Refactor the rows method in the statistics block

Divides the rows method to a small public rows method and a private
`row_lines` method. This allows further refactoring without having to
change the tests.

Use a map rather than an each in rows method to make the method
slightly smaller. `each_with_index` has been replaced with
`each_with_object` for the same reason.

The CSV rows are being returned with symbol keys to that `to_sym`
doesn't need to be used in multiple places.

Finally, the row keys and values are being obfuscated.

We can't guarantee what the keys will be named so it's better not to
rely on them.

We're still assuming that each row will have 3 columns:

- x-axis value
- variable / line name
- y-axis value

app/models/block/statistics.rb

@@ -25,30 +25,12 @@ def x_axis_keys
     end
 
     def rows
-      lines = {}
-      rows = []
-
-      csv_rows.each_with_index do |row, _i|
-        if lines.key?(row["variable"].to_sym)
-          lines[row["variable"].to_sym][:values] << row["value"].to_i
-        else
-          lines[row["variable"].to_sym] = {
-            label: row["Date"],
-            values: [
-              row["value"].to_i,
-            ],
-          }
-        end
-      end
-
-      lines.each_key do |key|
-        rows << {
+      row_lines.map do |key, value|
+        {
           label: key.to_s,
-          values: lines[key][:values],
+          values: value[:values],
         }
       end
-
-      rows
     end
 
     def data_source_link
@@ -57,8 +39,27 @@ def data_source_link
 
   private
 
+    def row_lines
+      csv_rows.each_with_object({}) do |row, lines|
+        label = row.keys.first
+        variable_name = row.values.second
+        value = row.values.last
+        if lines.key?(variable_name)
+          lines[variable_name][:values] << value.to_i
+        else
+          lines[variable_name] = {
+            label:,
+            values: [
+              value.to_i,
+            ],
+          }
+        end
+      end
+    end
+
     def csv_rows
-      CSV.read(csv_file_path, headers: true).map(&:to_h)
+      rows = CSV.read(csv_file_path, headers: true).map(&:to_h)
+      rows.each(&:deep_symbolize_keys!)
     end
 
     def csv_file_path