diff --git a/composer.json b/composer.json
index f448b6e..14213eb 100644
--- a/composer.json
+++ b/composer.json
@@ -1,6 +1,5 @@
 {
   "name": "iter8/builder",
-  "version": "3.0.3",
   "description": "A wrapper for the PHP Excel library to help you quickly build reports",
   "keywords": ["phpexcel", "reports", "excel", "spout", "silex"],
   "license": "MIT",
@@ -21,11 +20,13 @@
   "require": {
     "php": "^7.1",
     "phpoffice/phpspreadsheet": "~1.2",
-    "box/spout": "^2.7"
+    "box/spout": "^2.7",
+    "webmozart/assert": "^1.3"
   },
   "require-dev": {
     "phpunit/phpunit": "^7.0",
-    "silex/silex": "^2.0"
+    "silex/silex": "^2.0",
+    "symfony/var-dumper": "^4.1"
   },
   "autoload": {
     "psr-4": {
diff --git a/composer.lock b/composer.lock
index a7a7417..cb7734a 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "9fb19db6c0ba0126812375820b10989c",
+    "content-hash": "98557f66691846b17e7be45d59e73df0",
     "packages": [
         {
             "name": "box/spout",
@@ -74,18 +74,113 @@
             ],
             "time": "2017-09-25T19:44:35+00:00"
         },
+        {
+            "name": "markbaker/complex",
+            "version": "1.4.7",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/MarkBaker/PHPComplex.git",
+                "reference": "1ea674a8308baf547cbcbd30c5fcd6d301b7c000"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://github.com/gitapi/repos/MarkBaker/PHPComplex/zipball/1ea674a8308baf547cbcbd30c5fcd6d301b7c000",
+                "reference": "1ea674a8308baf547cbcbd30c5fcd6d301b7c000",
+                "shasum": ""
+            },
+            "require": {
+                "php": "^5.6.0|^7.0.0"
+            },
+            "require-dev": {
+                "dealerdirect/phpcodesniffer-composer-installer": "^0.4.3",
+                "phpcompatibility/php-compatibility": "^8.0",
+                "phpdocumentor/phpdocumentor": "2.*",
+                "phploc/phploc": "2.*",
+                "phpmd/phpmd": "2.*",
+                "phpunit/phpunit": "^4.8.35|^5.4.0",
+                "sebastian/phpcpd": "2.*",
+                "squizlabs/php_codesniffer": "^3.3.0"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Complex\\": "classes/src/"
+                },
+                "files": [
+                    "classes/src/functions/abs.php",
+                    "classes/src/functions/acos.php",
+                    "classes/src/functions/acosh.php",
+                    "classes/src/functions/acot.php",
+                    "classes/src/functions/acoth.php",
+                    "classes/src/functions/acsc.php",
+                    "classes/src/functions/acsch.php",
+                    "classes/src/functions/argument.php",
+                    "classes/src/functions/asec.php",
+                    "classes/src/functions/asech.php",
+                    "classes/src/functions/asin.php",
+                    "classes/src/functions/asinh.php",
+                    "classes/src/functions/atan.php",
+                    "classes/src/functions/atanh.php",
+                    "classes/src/functions/conjugate.php",
+                    "classes/src/functions/cos.php",
+                    "classes/src/functions/cosh.php",
+                    "classes/src/functions/cot.php",
+                    "classes/src/functions/coth.php",
+                    "classes/src/functions/csc.php",
+                    "classes/src/functions/csch.php",
+                    "classes/src/functions/exp.php",
+                    "classes/src/functions/inverse.php",
+                    "classes/src/functions/ln.php",
+                    "classes/src/functions/log2.php",
+                    "classes/src/functions/log10.php",
+                    "classes/src/functions/negative.php",
+                    "classes/src/functions/pow.php",
+                    "classes/src/functions/rho.php",
+                    "classes/src/functions/sec.php",
+                    "classes/src/functions/sech.php",
+                    "classes/src/functions/sin.php",
+                    "classes/src/functions/sinh.php",
+                    "classes/src/functions/sqrt.php",
+                    "classes/src/functions/tan.php",
+                    "classes/src/functions/tanh.php",
+                    "classes/src/functions/theta.php",
+                    "classes/src/operations/add.php",
+                    "classes/src/operations/subtract.php",
+                    "classes/src/operations/multiply.php",
+                    "classes/src/operations/divideby.php",
+                    "classes/src/operations/divideinto.php"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Mark Baker",
+                    "email": "mark@lange.demon.co.uk"
+                }
+            ],
+            "description": "PHP Class for working with complex numbers",
+            "homepage": "https://github.com/MarkBaker/PHPComplex",
+            "keywords": [
+                "complex",
+                "mathematics"
+            ],
+            "time": "2018-10-13T23:28:42+00:00"
+        },
         {
             "name": "phpoffice/phpspreadsheet",
-            "version": "1.3.1",
+            "version": "1.5.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/PHPOffice/PhpSpreadsheet.git",
-                "reference": "aa5b0d0236c907fd8dba0883f3ceb97cc52e46ec"
+                "reference": "2dfd06c59825914a1a325f2a2ed13634b9d8c411"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/PHPOffice/PhpSpreadsheet/zipball/aa5b0d0236c907fd8dba0883f3ceb97cc52e46ec",
-                "reference": "aa5b0d0236c907fd8dba0883f3ceb97cc52e46ec",
+                "url": "https://github.com/gitapi/repos/PHPOffice/PhpSpreadsheet/zipball/2dfd06c59825914a1a325f2a2ed13634b9d8c411",
+                "reference": "2dfd06c59825914a1a325f2a2ed13634b9d8c411",
                 "shasum": ""
             },
             "require": {
@@ -101,6 +196,7 @@
                 "ext-xmlwriter": "*",
                 "ext-zip": "*",
                 "ext-zlib": "*",
+                "markbaker/complex": "^1.4.1",
                 "php": "^5.6|^7.0",
                 "psr/simple-cache": "^1.0"
             },
@@ -110,14 +206,14 @@
                 "jpgraph/jpgraph": "^4.0",
                 "mpdf/mpdf": "^7.0.0",
                 "phpunit/phpunit": "^5.7",
-                "squizlabs/php_codesniffer": "^2.7",
+                "squizlabs/php_codesniffer": "^3.3",
                 "tecnickcom/tcpdf": "^6.2"
             },
             "suggest": {
                 "dompdf/dompdf": "Option for rendering PDF with PDF Writer",
                 "jpgraph/jpgraph": "Option for rendering charts, or including charts with PDF or HTML Writers",
                 "mpdf/mpdf": "Option for rendering PDF with PDF Writer",
-                "tecnick.com/tcpdf": "Option for rendering PDF with PDF Writer"
+                "tecnickcom/tcpdf": "Option for rendering PDF with PDF Writer"
             },
             "type": "library",
             "autoload": {
@@ -158,7 +254,7 @@
                 "xls",
                 "xlsx"
             ],
-            "time": "2018-06-12T13:40:21+00:00"
+            "time": "2018-10-21T10:04:54+00:00"
         },
         {
             "name": "psr/simple-cache",
@@ -207,6 +303,56 @@
                 "simple-cache"
             ],
             "time": "2017-10-23T01:57:42+00:00"
+        },
+        {
+            "name": "webmozart/assert",
+            "version": "1.3.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/webmozart/assert.git",
+                "reference": "0df1908962e7a3071564e857d86874dad1ef204a"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://github.com/gitapi/repos/webmozart/assert/zipball/0df1908962e7a3071564e857d86874dad1ef204a",
+                "reference": "0df1908962e7a3071564e857d86874dad1ef204a",
+                "shasum": ""
+            },
+            "require": {
+                "php": "^5.3.3 || ^7.0"
+            },
+            "require-dev": {
+                "phpunit/phpunit": "^4.6",
+                "sebastian/version": "^1.0.1"
+            },
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "1.3-dev"
+                }
+            },
+            "autoload": {
+                "psr-4": {
+                    "Webmozart\\Assert\\": "src/"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Bernhard Schussek",
+                    "email": "bschussek@gmail.com"
+                }
+            ],
+            "description": "Assertions to validate method input/output with nice error messages.",
+            "keywords": [
+                "assert",
+                "check",
+                "validate"
+            ],
+            "time": "2018-01-29T19:49:41+00:00"
         }
     ],
     "packages-dev": [
@@ -568,16 +714,16 @@
         },
         {
             "name": "phpspec/prophecy",
-            "version": "1.7.6",
+            "version": "1.8.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/phpspec/prophecy.git",
-                "reference": "33a7e3c4fda54e912ff6338c48823bd5c0f0b712"
+                "reference": "4ba436b55987b4bf311cb7c6ba82aa528aac0a06"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/phpspec/prophecy/zipball/33a7e3c4fda54e912ff6338c48823bd5c0f0b712",
-                "reference": "33a7e3c4fda54e912ff6338c48823bd5c0f0b712",
+                "url": "https://github.com/gitapi/repos/phpspec/prophecy/zipball/4ba436b55987b4bf311cb7c6ba82aa528aac0a06",
+                "reference": "4ba436b55987b4bf311cb7c6ba82aa528aac0a06",
                 "shasum": ""
             },
             "require": {
@@ -589,12 +735,12 @@
             },
             "require-dev": {
                 "phpspec/phpspec": "^2.5|^3.2",
-                "phpunit/phpunit": "^4.8.35 || ^5.7 || ^6.5"
+                "phpunit/phpunit": "^4.8.35 || ^5.7 || ^6.5 || ^7.1"
             },
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "1.7.x-dev"
+                    "dev-master": "1.8.x-dev"
                 }
             },
             "autoload": {
@@ -627,20 +773,20 @@
                 "spy",
                 "stub"
             ],
-            "time": "2018-04-18T13:57:24+00:00"
+            "time": "2018-08-05T17:53:17+00:00"
         },
         {
             "name": "phpunit/php-code-coverage",
-            "version": "6.0.7",
+            "version": "6.1.3",
             "source": {
                 "type": "git",
                 "url": "https://github.com/sebastianbergmann/php-code-coverage.git",
-                "reference": "865662550c384bc1db7e51d29aeda1c2c161d69a"
+                "reference": "4d3ae9b21a7d7e440bd0cf65565533117976859f"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/sebastianbergmann/php-code-coverage/zipball/865662550c384bc1db7e51d29aeda1c2c161d69a",
-                "reference": "865662550c384bc1db7e51d29aeda1c2c161d69a",
+                "url": "https://github.com/gitapi/repos/sebastianbergmann/php-code-coverage/zipball/4d3ae9b21a7d7e440bd0cf65565533117976859f",
+                "reference": "4d3ae9b21a7d7e440bd0cf65565533117976859f",
                 "shasum": ""
             },
             "require": {
@@ -651,7 +797,7 @@
                 "phpunit/php-text-template": "^1.2.1",
                 "phpunit/php-token-stream": "^3.0",
                 "sebastian/code-unit-reverse-lookup": "^1.0.1",
-                "sebastian/environment": "^3.1",
+                "sebastian/environment": "^3.1 || ^4.0",
                 "sebastian/version": "^2.0.1",
                 "theseer/tokenizer": "^1.1"
             },
@@ -664,7 +810,7 @@
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "6.0-dev"
+                    "dev-master": "6.1-dev"
                 }
             },
             "autoload": {
@@ -690,25 +836,28 @@
                 "testing",
                 "xunit"
             ],
-            "time": "2018-06-01T07:51:50+00:00"
+            "time": "2018-10-23T05:59:32+00:00"
         },
         {
             "name": "phpunit/php-file-iterator",
-            "version": "2.0.1",
+            "version": "2.0.2",
             "source": {
                 "type": "git",
                 "url": "https://github.com/sebastianbergmann/php-file-iterator.git",
-                "reference": "cecbc684605bb0cc288828eb5d65d93d5c676d3c"
+                "reference": "050bedf145a257b1ff02746c31894800e5122946"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/sebastianbergmann/php-file-iterator/zipball/cecbc684605bb0cc288828eb5d65d93d5c676d3c",
-                "reference": "cecbc684605bb0cc288828eb5d65d93d5c676d3c",
+                "url": "https://github.com/gitapi/repos/sebastianbergmann/php-file-iterator/zipball/050bedf145a257b1ff02746c31894800e5122946",
+                "reference": "050bedf145a257b1ff02746c31894800e5122946",
                 "shasum": ""
             },
             "require": {
                 "php": "^7.1"
             },
+            "require-dev": {
+                "phpunit/phpunit": "^7.1"
+            },
             "type": "library",
             "extra": {
                 "branch-alias": {
@@ -737,7 +886,7 @@
                 "filesystem",
                 "iterator"
             ],
-            "time": "2018-06-11T11:44:00+00:00"
+            "time": "2018-09-13T20:33:42+00:00"
         },
         {
             "name": "phpunit/php-text-template",
@@ -880,16 +1029,16 @@
         },
         {
             "name": "phpunit/phpunit",
-            "version": "7.2.7",
+            "version": "7.4.3",
             "source": {
                 "type": "git",
                 "url": "https://github.com/sebastianbergmann/phpunit.git",
-                "reference": "8e878aff7917ef66e702e03d1359b16eee254e2c"
+                "reference": "c151651fb6ed264038d486ea262e243af72e5e64"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/sebastianbergmann/phpunit/zipball/8e878aff7917ef66e702e03d1359b16eee254e2c",
-                "reference": "8e878aff7917ef66e702e03d1359b16eee254e2c",
+                "url": "https://github.com/gitapi/repos/sebastianbergmann/phpunit/zipball/c151651fb6ed264038d486ea262e243af72e5e64",
+                "reference": "c151651fb6ed264038d486ea262e243af72e5e64",
                 "shasum": ""
             },
             "require": {
@@ -910,11 +1059,11 @@
                 "phpunit/php-timer": "^2.0",
                 "sebastian/comparator": "^3.0",
                 "sebastian/diff": "^3.0",
-                "sebastian/environment": "^3.1",
+                "sebastian/environment": "^3.1 || ^4.0",
                 "sebastian/exporter": "^3.1",
                 "sebastian/global-state": "^2.0",
                 "sebastian/object-enumerator": "^3.0.3",
-                "sebastian/resource-operations": "^1.0",
+                "sebastian/resource-operations": "^2.0",
                 "sebastian/version": "^2.0.1"
             },
             "conflict": {
@@ -934,7 +1083,7 @@
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "7.2-dev"
+                    "dev-master": "7.4-dev"
                 }
             },
             "autoload": {
@@ -960,7 +1109,7 @@
                 "testing",
                 "xunit"
             ],
-            "time": "2018-07-15T05:20:50+00:00"
+            "time": "2018-10-23T05:57:41+00:00"
         },
         {
             "name": "pimple/pimple",
@@ -1588,25 +1737,25 @@
         },
         {
             "name": "sebastian/resource-operations",
-            "version": "1.0.0",
+            "version": "2.0.1",
             "source": {
                 "type": "git",
                 "url": "https://github.com/sebastianbergmann/resource-operations.git",
-                "reference": "ce990bb21759f94aeafd30209e8cfcdfa8bc3f52"
+                "reference": "4d7a795d35b889bf80a0cc04e08d77cedfa917a9"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/sebastianbergmann/resource-operations/zipball/ce990bb21759f94aeafd30209e8cfcdfa8bc3f52",
-                "reference": "ce990bb21759f94aeafd30209e8cfcdfa8bc3f52",
+                "url": "https://github.com/gitapi/repos/sebastianbergmann/resource-operations/zipball/4d7a795d35b889bf80a0cc04e08d77cedfa917a9",
+                "reference": "4d7a795d35b889bf80a0cc04e08d77cedfa917a9",
                 "shasum": ""
             },
             "require": {
-                "php": ">=5.6.0"
+                "php": "^7.1"
             },
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "1.0.x-dev"
+                    "dev-master": "2.0-dev"
                 }
             },
             "autoload": {
@@ -1626,7 +1775,7 @@
             ],
             "description": "Provides a list of PHP built-in functions that operate on resources",
             "homepage": "https://www.github.com/sebastianbergmann/resource-operations",
-            "time": "2015-07-28T20:34:47+00:00"
+            "time": "2018-10-04T04:07:39+00:00"
         },
         {
             "name": "sebastian/version",
@@ -1755,20 +1904,21 @@
             "keywords": [
                 "microframework"
             ],
+            "abandoned": "symfony/flex",
             "time": "2018-04-20T05:17:01+00:00"
         },
         {
             "name": "symfony/debug",
-            "version": "v4.1.2",
+            "version": "v4.1.6",
             "source": {
                 "type": "git",
                 "url": "https://github.com/symfony/debug.git",
-                "reference": "a1f2118cedb8731c45e945cdd2b808ca82abc4b5"
+                "reference": "e3f76ce6198f81994e019bb2b4e533e9de1b9b90"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/symfony/debug/zipball/a1f2118cedb8731c45e945cdd2b808ca82abc4b5",
-                "reference": "a1f2118cedb8731c45e945cdd2b808ca82abc4b5",
+                "url": "https://github.com/gitapi/repos/symfony/debug/zipball/e3f76ce6198f81994e019bb2b4e533e9de1b9b90",
+                "reference": "e3f76ce6198f81994e019bb2b4e533e9de1b9b90",
                 "shasum": ""
             },
             "require": {
@@ -1811,20 +1961,20 @@
             ],
             "description": "Symfony Debug Component",
             "homepage": "https://symfony.com",
-            "time": "2018-07-06T14:52:28+00:00"
+            "time": "2018-10-02T16:36:10+00:00"
         },
         {
             "name": "symfony/event-dispatcher",
-            "version": "v4.1.2",
+            "version": "v4.1.6",
             "source": {
                 "type": "git",
                 "url": "https://github.com/symfony/event-dispatcher.git",
-                "reference": "00d64638e4f0703a00ab7fc2c8ae5f75f3b4020f"
+                "reference": "bfb30c2ad377615a463ebbc875eba64a99f6aa3e"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/symfony/event-dispatcher/zipball/00d64638e4f0703a00ab7fc2c8ae5f75f3b4020f",
-                "reference": "00d64638e4f0703a00ab7fc2c8ae5f75f3b4020f",
+                "url": "https://github.com/gitapi/repos/symfony/event-dispatcher/zipball/bfb30c2ad377615a463ebbc875eba64a99f6aa3e",
+                "reference": "bfb30c2ad377615a463ebbc875eba64a99f6aa3e",
                 "shasum": ""
             },
             "require": {
@@ -1874,20 +2024,20 @@
             ],
             "description": "Symfony EventDispatcher Component",
             "homepage": "https://symfony.com",
-            "time": "2018-07-10T11:02:47+00:00"
+            "time": "2018-07-26T09:10:45+00:00"
         },
         {
             "name": "symfony/http-foundation",
-            "version": "v4.1.2",
+            "version": "v4.1.6",
             "source": {
                 "type": "git",
                 "url": "https://github.com/symfony/http-foundation.git",
-                "reference": "8da9ea68ab2d80dfabd41e0d14b9606bb47a10c0"
+                "reference": "d528136617ff24f530e70df9605acc1b788b08d4"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/symfony/http-foundation/zipball/8da9ea68ab2d80dfabd41e0d14b9606bb47a10c0",
-                "reference": "8da9ea68ab2d80dfabd41e0d14b9606bb47a10c0",
+                "url": "https://github.com/gitapi/repos/symfony/http-foundation/zipball/d528136617ff24f530e70df9605acc1b788b08d4",
+                "reference": "d528136617ff24f530e70df9605acc1b788b08d4",
                 "shasum": ""
             },
             "require": {
@@ -1928,20 +2078,20 @@
             ],
             "description": "Symfony HttpFoundation Component",
             "homepage": "https://symfony.com",
-            "time": "2018-07-16T14:05:40+00:00"
+            "time": "2018-10-03T08:48:45+00:00"
         },
         {
             "name": "symfony/http-kernel",
-            "version": "v4.1.2",
+            "version": "v4.1.6",
             "source": {
                 "type": "git",
                 "url": "https://github.com/symfony/http-kernel.git",
-                "reference": "ebd28f4f88a2ca0a0488882ad73c4004f3afdbe3"
+                "reference": "f5e7c15a5d010be0e16ce798594c5960451d4220"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/symfony/http-kernel/zipball/ebd28f4f88a2ca0a0488882ad73c4004f3afdbe3",
-                "reference": "ebd28f4f88a2ca0a0488882ad73c4004f3afdbe3",
+                "url": "https://github.com/gitapi/repos/symfony/http-kernel/zipball/f5e7c15a5d010be0e16ce798594c5960451d4220",
+                "reference": "f5e7c15a5d010be0e16ce798594c5960451d4220",
                 "shasum": ""
             },
             "require": {
@@ -2015,29 +2165,32 @@
             ],
             "description": "Symfony HttpKernel Component",
             "homepage": "https://symfony.com",
-            "time": "2018-07-23T17:16:22+00:00"
+            "time": "2018-10-03T12:53:38+00:00"
         },
         {
             "name": "symfony/polyfill-ctype",
-            "version": "v1.8.0",
+            "version": "v1.9.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/symfony/polyfill-ctype.git",
-                "reference": "7cc359f1b7b80fc25ed7796be7d96adc9b354bae"
+                "reference": "e3d826245268269cd66f8326bd8bc066687b4a19"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/symfony/polyfill-ctype/zipball/7cc359f1b7b80fc25ed7796be7d96adc9b354bae",
-                "reference": "7cc359f1b7b80fc25ed7796be7d96adc9b354bae",
+                "url": "https://github.com/gitapi/repos/symfony/polyfill-ctype/zipball/e3d826245268269cd66f8326bd8bc066687b4a19",
+                "reference": "e3d826245268269cd66f8326bd8bc066687b4a19",
                 "shasum": ""
             },
             "require": {
                 "php": ">=5.3.3"
             },
+            "suggest": {
+                "ext-ctype": "For best performance"
+            },
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "1.8-dev"
+                    "dev-master": "1.9-dev"
                 }
             },
             "autoload": {
@@ -2070,20 +2223,20 @@
                 "polyfill",
                 "portable"
             ],
-            "time": "2018-04-30T19:57:29+00:00"
+            "time": "2018-08-06T14:22:27+00:00"
         },
         {
             "name": "symfony/polyfill-mbstring",
-            "version": "v1.8.0",
+            "version": "v1.9.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/symfony/polyfill-mbstring.git",
-                "reference": "3296adf6a6454a050679cde90f95350ad604b171"
+                "reference": "d0cd638f4634c16d8df4508e847f14e9e43168b8"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/symfony/polyfill-mbstring/zipball/3296adf6a6454a050679cde90f95350ad604b171",
-                "reference": "3296adf6a6454a050679cde90f95350ad604b171",
+                "url": "https://github.com/gitapi/repos/symfony/polyfill-mbstring/zipball/d0cd638f4634c16d8df4508e847f14e9e43168b8",
+                "reference": "d0cd638f4634c16d8df4508e847f14e9e43168b8",
                 "shasum": ""
             },
             "require": {
@@ -2095,7 +2248,7 @@
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "1.8-dev"
+                    "dev-master": "1.9-dev"
                 }
             },
             "autoload": {
@@ -2129,20 +2282,75 @@
                 "portable",
                 "shim"
             ],
-            "time": "2018-04-26T10:06:28+00:00"
+            "time": "2018-08-06T14:22:27+00:00"
+        },
+        {
+            "name": "symfony/polyfill-php72",
+            "version": "v1.9.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/symfony/polyfill-php72.git",
+                "reference": "95c50420b0baed23852452a7f0c7b527303ed5ae"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://github.com/gitapi/repos/symfony/polyfill-php72/zipball/95c50420b0baed23852452a7f0c7b527303ed5ae",
+                "reference": "95c50420b0baed23852452a7f0c7b527303ed5ae",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=5.3.3"
+            },
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "1.9-dev"
+                }
+            },
+            "autoload": {
+                "psr-4": {
+                    "Symfony\\Polyfill\\Php72\\": ""
+                },
+                "files": [
+                    "bootstrap.php"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Nicolas Grekas",
+                    "email": "p@tchwork.com"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
+                }
+            ],
+            "description": "Symfony polyfill backporting some PHP 7.2+ features to lower PHP versions",
+            "homepage": "https://symfony.com",
+            "keywords": [
+                "compatibility",
+                "polyfill",
+                "portable",
+                "shim"
+            ],
+            "time": "2018-08-06T14:22:27+00:00"
         },
         {
             "name": "symfony/routing",
-            "version": "v4.1.2",
+            "version": "v4.1.6",
             "source": {
                 "type": "git",
                 "url": "https://github.com/symfony/routing.git",
-                "reference": "73770bf3682b4407b017c2bdcb2b11cdcbce5322"
+                "reference": "537803f0bdfede36b9acef052d2e4d447d9fa0e9"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/symfony/routing/zipball/73770bf3682b4407b017c2bdcb2b11cdcbce5322",
-                "reference": "73770bf3682b4407b017c2bdcb2b11cdcbce5322",
+                "url": "https://github.com/gitapi/repos/symfony/routing/zipball/537803f0bdfede36b9acef052d2e4d447d9fa0e9",
+                "reference": "537803f0bdfede36b9acef052d2e4d447d9fa0e9",
                 "shasum": ""
             },
             "require": {
@@ -2206,97 +2414,122 @@
                 "uri",
                 "url"
             ],
-            "time": "2018-06-28T06:30:33+00:00"
+            "time": "2018-10-02T12:40:59+00:00"
         },
         {
-            "name": "theseer/tokenizer",
-            "version": "1.1.0",
+            "name": "symfony/var-dumper",
+            "version": "v4.1.6",
             "source": {
                 "type": "git",
-                "url": "https://github.com/theseer/tokenizer.git",
-                "reference": "cb2f008f3f05af2893a87208fe6a6c4985483f8b"
+                "url": "https://github.com/symfony/var-dumper.git",
+                "reference": "60319b45653580b0cdacca499344577d87732f16"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/theseer/tokenizer/zipball/cb2f008f3f05af2893a87208fe6a6c4985483f8b",
-                "reference": "cb2f008f3f05af2893a87208fe6a6c4985483f8b",
+                "url": "https://github.com/gitapi/repos/symfony/var-dumper/zipball/60319b45653580b0cdacca499344577d87732f16",
+                "reference": "60319b45653580b0cdacca499344577d87732f16",
                 "shasum": ""
             },
             "require": {
-                "ext-dom": "*",
-                "ext-tokenizer": "*",
-                "ext-xmlwriter": "*",
-                "php": "^7.0"
+                "php": "^7.1.3",
+                "symfony/polyfill-mbstring": "~1.0",
+                "symfony/polyfill-php72": "~1.5"
+            },
+            "conflict": {
+                "phpunit/phpunit": "<4.8.35|<5.4.3,>=5.0",
+                "symfony/console": "<3.4"
+            },
+            "require-dev": {
+                "ext-iconv": "*",
+                "symfony/process": "~3.4|~4.0",
+                "twig/twig": "~1.34|~2.4"
             },
+            "suggest": {
+                "ext-iconv": "To convert non-UTF-8 strings to UTF-8 (or symfony/polyfill-iconv in case ext-iconv cannot be used).",
+                "ext-intl": "To show region name in time zone dump",
+                "symfony/console": "To use the ServerDumpCommand and/or the bin/var-dump-server script"
+            },
+            "bin": [
+                "Resources/bin/var-dump-server"
+            ],
             "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "4.1-dev"
+                }
+            },
             "autoload": {
-                "classmap": [
-                    "src/"
+                "files": [
+                    "Resources/functions/dump.php"
+                ],
+                "psr-4": {
+                    "Symfony\\Component\\VarDumper\\": ""
+                },
+                "exclude-from-classmap": [
+                    "/Tests/"
                 ]
             },
             "notification-url": "https://packagist.org/downloads/",
             "license": [
-                "BSD-3-Clause"
+                "MIT"
             ],
             "authors": [
                 {
-                    "name": "Arne Blankerts",
-                    "email": "arne@blankerts.de",
-                    "role": "Developer"
+                    "name": "Nicolas Grekas",
+                    "email": "p@tchwork.com"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
                 }
             ],
-            "description": "A small library for converting tokenized PHP source code into XML and potentially other formats",
-            "time": "2017-04-07T12:08:54+00:00"
+            "description": "Symfony mechanism for exploring and dumping PHP variables",
+            "homepage": "https://symfony.com",
+            "keywords": [
+                "debug",
+                "dump"
+            ],
+            "time": "2018-10-02T16:36:10+00:00"
         },
         {
-            "name": "webmozart/assert",
-            "version": "1.3.0",
+            "name": "theseer/tokenizer",
+            "version": "1.1.0",
             "source": {
                 "type": "git",
-                "url": "https://github.com/webmozart/assert.git",
-                "reference": "0df1908962e7a3071564e857d86874dad1ef204a"
+                "url": "https://github.com/theseer/tokenizer.git",
+                "reference": "cb2f008f3f05af2893a87208fe6a6c4985483f8b"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://github.com/gitapi/repos/webmozart/assert/zipball/0df1908962e7a3071564e857d86874dad1ef204a",
-                "reference": "0df1908962e7a3071564e857d86874dad1ef204a",
+                "url": "https://github.com/gitapi/repos/theseer/tokenizer/zipball/cb2f008f3f05af2893a87208fe6a6c4985483f8b",
+                "reference": "cb2f008f3f05af2893a87208fe6a6c4985483f8b",
                 "shasum": ""
             },
             "require": {
-                "php": "^5.3.3 || ^7.0"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "^4.6",
-                "sebastian/version": "^1.0.1"
+                "ext-dom": "*",
+                "ext-tokenizer": "*",
+                "ext-xmlwriter": "*",
+                "php": "^7.0"
             },
             "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.3-dev"
-                }
-            },
             "autoload": {
-                "psr-4": {
-                    "Webmozart\\Assert\\": "src/"
-                }
+                "classmap": [
+                    "src/"
+                ]
             },
             "notification-url": "https://packagist.org/downloads/",
             "license": [
-                "MIT"
+                "BSD-3-Clause"
             ],
             "authors": [
                 {
-                    "name": "Bernhard Schussek",
-                    "email": "bschussek@gmail.com"
+                    "name": "Arne Blankerts",
+                    "email": "arne@blankerts.de",
+                    "role": "Developer"
                 }
             ],
-            "description": "Assertions to validate method input/output with nice error messages.",
-            "keywords": [
-                "assert",
-                "check",
-                "validate"
-            ],
-            "time": "2018-01-29T19:49:41+00:00"
+            "description": "A small library for converting tokenized PHP source code into XML and potentially other formats",
+            "time": "2017-04-07T12:08:54+00:00"
         }
     ],
     "aliases": [],
diff --git a/src/Builder/Builder.php b/src/Builder/Builder.php
index f62160f..5210294 100644
--- a/src/Builder/Builder.php
+++ b/src/Builder/Builder.php
@@ -41,7 +41,12 @@ class Builder
     /**
      * @var array
      */
-    private $data;
+    private $headers = [];
+
+    /**
+     * @var array
+     */
+    private $data = [];
 
     /**
      * @var null|string
@@ -53,7 +58,10 @@ class Builder
      */
     private $title;
 
-    private $sheetTitles;
+    /**
+     * @var array
+     */
+    private $sheetTitles = [];
 
     /**
      * @var null|string
@@ -68,17 +76,17 @@ class Builder
     /**
      * @var array
      */
-    private $sheets;
+    private $sheets = [];
 
     /**
      * @var array
      */
-    private $columnWidths;
+    private $columnWidths = [];
 
     /**
      * @var array
      */
-    private $columnStyles;
+    private $columnStyles = [];
 
     /**
      * Builder constructor.
@@ -171,12 +179,14 @@ public function generateExcel(): void
             $this->createSheets();
         } else {
             // Single sheet - these will be an array and a string.
+            $headers     = $this->getHeaders();
             $reportArray = $this->getData();
             $sheetTitle  = $this->getSheetTitles();
 
             $this->builder->setActiveSheetIndex(0);
 
             $this->createSheet(
+                $headers,
                 $reportArray,
                 $sheetTitle
             );
@@ -210,7 +220,8 @@ public function createSheets(): void
 
         foreach ($sheets as $sheet) {
             $this->createSheet(
-                $sheet,
+                $sheet['headers'],
+                $sheet['rows'],
                 $titles[$sheetCount]
             );
 
@@ -231,13 +242,15 @@ public function createSheets(): void
     }
 
     /**
-     * @param array $data
+     * @param array  $headers
+     * @param array  $rows
      * @param string $title
      *
      * @return void
      */
     public function createSheet(
-        array $data,
+        array $headers,
+        array $rows,
         string $title
     ): void {
         // Check if we are setting any custom column widths.
@@ -261,14 +274,14 @@ public function createSheet(
         ]);
 
         // Build column headers.
-        $this->builder->buildHeaderRow($data[0], $style);
+        $this->builder->buildHeaderRow($headers, $style);
 
         // Build all the rows now.
-        $this->builder->buildRows($data);
+        $this->builder->buildRows($rows);
 
         // If no column widths are specified, then simply auto-size all columns.
         if (!$this->hasColumnWidths()) {
-            $this->builder->autoSizeColumns($data[0]);
+            $this->builder->autoSizeColumns($rows[0]);
         }
 
         // Rename sheet.
@@ -324,19 +337,23 @@ public function setCreator($creator): self
         return $this;
     }
 
-    /**
-     * @return array
-     */
+    public function getHeaders(): array
+    {
+        return $this->headers;
+    }
+
+    public function setHeaders(array $headers): self
+    {
+        $this->headers = $headers;
+
+        return $this;
+    }
+
     public function getData(): array
     {
         return $this->data;
     }
 
-    /**
-     * @param array $data
-     *
-     * @return $this
-     */
     public function setData(array $data): self
     {
         $this->data = $data;
diff --git a/src/Builder/Builders/SpoutBuilder.php b/src/Builder/Builders/SpoutBuilder.php
index dad9f83..4871f69 100644
--- a/src/Builder/Builders/SpoutBuilder.php
+++ b/src/Builder/Builders/SpoutBuilder.php
@@ -181,12 +181,10 @@ public function buildHeaderRow(
             $this->initialise();
         }
 
-        $keys = array_keys($columns);
-
         if ($style instanceof Style) {
-            $this->writer->addRowWithStyle($keys, $style);
+            $this->writer->addRowWithStyle($columns, $style);
         } else {
-            $this->writer->addRow($keys);
+            $this->writer->addRow($columns);
         }
 
         return;
diff --git a/tests/PHPSpreadsheetTest.php b/tests/PHPSpreadsheetTest.php
index d8797de..5062853 100644
--- a/tests/PHPSpreadsheetTest.php
+++ b/tests/PHPSpreadsheetTest.php
@@ -45,22 +45,29 @@ public function can_create_single_sheet_spreadsheet()
 
         // Act
         $app['builder']->setSheetTitles('PHPExcel Test');
+        $app['builder']->setHeaders(
+            [
+                'Column 1',
+                'Column 2',
+                'Column 3',
+            ]
+        );
         $app['builder']->setData(
             [
                 [
-                    'Column 1' => 'column_1',
-                    'Column 2' => 'column_2',
-                    'Column 3' => 'column_3',
+                    'column_1',
+                    'column_2',
+                    'column_3',
                 ],
                 [
-                    'Column 1' => '1',
-                    'Column 2' => 'Two',
-                    'Column 3' => '333'
+                    '1',
+                    'Two',
+                    '333'
                 ],
                 [
-                    'Column 1' => 'One',
-                    'Column 2' => '2',
-                    'Column 3' => 'Three x 3'
+                    'One',
+                    '2',
+                    'Three x 3'
                 ],
             ]
         );
@@ -95,20 +102,32 @@ public function can_create_multi_sheet_spreadsheet()
         $app['builder']->setSheets(
             [
                 [
-                    [
-                        'Column 1' => 'Row 1',
-                        'Column 2' => 'Sheet 1',
+                    'headers' => [
+                        'Column 1',
+                        'Column 2',
+                    ],
+                    'rows' => [
+                        [
+                            'Row 1',
+                            'Sheet 1',
+                        ]
                     ],
                 ],
                 [
-                    [
-                        'Column 1' => 'Row 2',
-                        'Column 2' => 'Sheet 2',
-                    ],
-                    [
-                        'Column 1' => 'Row 3',
-                        'Column 2' => 'Sheet 2',
+                    'headers' => [
+                        'Column 1',
+                        'Column 2',
                     ],
+                    'rows' => [
+                        [
+                            'Row 2',
+                            'Sheet 2',
+                        ],
+                        [
+                            'Row 3',
+                            'Sheet 2',
+                        ],
+                    ]
                 ],
             ]
         );
diff --git a/tests/SpoutTest.php b/tests/SpoutTest.php
index e2ffc4a..7a16c4f 100644
--- a/tests/SpoutTest.php
+++ b/tests/SpoutTest.php
@@ -3,6 +3,8 @@
 namespace Tests\Builder;
 
 use Box\Spout\Common\Helper\FileSystemHelper;
+use Box\Spout\Common\Type;
+use Box\Spout\Reader\ReaderFactory;
 use Box\Spout\Writer\Style\Color;
 use Box\Spout\Writer\Style\Style;
 use Box\Spout\Writer\Style\StyleBuilder;
@@ -72,8 +74,6 @@ public function row_style_is_parsed_correctly()
 
     /**
      * @test
-     *
-     * TODO: Open the file with the Reader and verify a row/column value.
      */
     public function can_create_single_sheet_spreadsheet()
     {
@@ -83,27 +83,20 @@ public function can_create_single_sheet_spreadsheet()
             'builder.default'   => 'spout',
             'builder.cache_dir' => $this->getCacheDir(),
         ]);
-        // $reader = ReaderFactory::create(Type::XLSX);
+        $reader = ReaderFactory::create(Type::XLSX);
 
         // Act
         $app['builder']->setSheetTitles('Spout Test');
+        $app['builder']->setHeaders([
+            'Column 1',
+            'Column 2',
+            'Column 3',
+        ]);
         $app['builder']->setData(
             [
-                [
-                    'Column 1' => 'column_1',
-                    'Column 2' => 'column_2',
-                    'Column 3' => 'column_3',
-                ],
-                [
-                    'Column 1' => '1',
-                    'Column 2' => 'Two',
-                    'Column 3' => '333'
-                ],
-                [
-                    'Column 1' => 'One',
-                    'Column 2' => '2',
-                    'Column 3' => 'Three x 3'
-                ],
+                ['column_1', 'column_2', 'column_3'],
+                ['1', 'Two', '333'],
+                ['One', '2', 'Three x 3'],
             ]
         );
         $app['builder']->generateExcel();
@@ -113,14 +106,36 @@ public function can_create_single_sheet_spreadsheet()
         // Assert
         $this->assertFileExists($generatedExcelFile);
         $this->assertGreaterThan(3000, stat($generatedExcelFile)['size']);
-//        $reader->open($generatedExcel);
-//
-//        $sheets = $reader->getSheetIterator();
-//
-//        $sheets->rewind();
-//
-//        $sheet = $sheets->current()->getRowIterator()->next()->current();
-//        die(var_dump($sheet));
+        $reader->open($generatedExcelFile);
+
+        // Need to use `iterator_to_array` as it's the only way to coerce Spout to read the spreadsheet into memory
+        // unless you manually do a foreach over the rows.
+        $sheets = iterator_to_array($reader->getSheetIterator());
+        // Sheets array is *NOT* zero-based when fetched from the iterator.
+        $sheetData = iterator_to_array($sheets[1]->getRowIterator());
+
+        $headers = array_shift($sheetData);
+        $rows = $sheetData;
+
+        $this->assertCount(3, $headers, sprintf('Headers row should have 3 values, "%d" supplied.', count($headers)));
+        $this->assertEquals('Column 1', $headers[0]);
+        $this->assertEquals('Column 2', $headers[1]);
+        $this->assertEquals('Column 3', $headers[2]);
+
+        $this->assertCount(3, $rows, sprintf('Rows should have 3 rows, "%d" supplied.', count($rows)));
+
+        $row1 = $rows[0];
+        $row3 = $rows[2];
+
+        $this->assertCount(3, $row1, sprintf('Row 1 should have 3 values, "%d" supplied.', count($row1)));
+        $this->assertEquals('column_1', $row1[0]);
+        $this->assertEquals('column_2', $row1[1]);
+        $this->assertEquals('column_3', $row1[2]);
+
+        $this->assertCount(3, $row3, sprintf('Row 3 should have 3 values, "%d" supplied.', count($row3)));
+        $this->assertEquals('One', $row3[0]);
+        $this->assertEquals('2', $row3[1]);
+        $this->assertEquals('Three x 3', $row3[2]);
     }
 
     /**
@@ -145,20 +160,32 @@ public function can_create_multi_sheet_spreadsheet()
         $app['builder']->setSheets(
             [
                 [
-                    [
-                        'Column 1' => 'Row 1',
-                        'Column 2' => 'Sheet 1',
+                    'headers' => [
+                        'Column 1',
+                        'Column 2',
+                     ],
+                    'rows' => [
+                        [
+                            'Row 1',
+                            'Sheet 1',
+                        ]
                     ],
                 ],
                 [
-                    [
-                        'Column 1' => 'Row 2',
-                        'Column 2' => 'Sheet 2',
-                    ],
-                    [
-                        'Column 1' => 'Row 3',
-                        'Column 2' => 'Sheet 2',
+                    'headers' => [
+                        'Column 1',
+                        'Column 2',
                     ],
+                    'rows' => [
+                        [
+                            'Row 2',
+                            'Sheet 2',
+                        ],
+                        [
+                            'Row 3',
+                            'Sheet 2',
+                        ],
+                    ]
                 ],
             ]
         );