In general, a treemap is a graphical method to visualize a hierarchical structure, also known as a tree. It shows the whole tree at once with each item of the tree coresponding to a rectangle in the treemap.
What makes the treemap special is that the area (the size) of each rectangle corresponds to a value of the item, so you can instantly see how any rectangle compares to the others.
In the context of disk space, that value is of course the disk space used by that item.
But it's not limited to that: For example, in a treemap of countries and their respective military spending, the area of a treemap item can correspond to that military spending.
A sales organization could use it to visualize the revenue by business units, departments and teams; or by product groups and products.
It is important to note that the grouping follows the hierarchy, so the business units or departments remain visible: Their total revenue is the total of their subunits which are clustered together.
Main window screenshot: Tree view (upper half), treemap (below), details panel (right).
QDirStat is about disk usage, so it uses the disk usage (the size) of each file or directory for the area of each treemap rectangle (tile).
But showing the sizes is not all: The treemap also uses colors to show the type (the MIME category) of each file.
By default, documents are displayed in blue, images in cyan, music in yellow, videos in light green; executable files in magenta, libraries in orange etc., but you can configure that to your own liking: The number of categories and what you want them to represent, the filename extensions, the color. See menu "Edit" -> "Configure" -> tab "MIME Categories".
That means that you can get a rough idea how your disk space is used just by looking at the colors.
If you spot something that makes you curious what it is (for example a particularly large rectangle), click on it: It will be selected in the treemap (notice the red frame around it), in the tree view above (it will scroll to that item and open its parent branches), and the details panel on the right will show details about it.
It works the same when you select an item in the tree view: The corresponding treemap tile will get the red selection frame, and the details panel will show details about it.
When an item is selected, no matter in which view (the tree view or the treemap), you can start actions on it: Right-click to open the context menu, or use one of the small icons in the toolbar below the main menu menu; or one of the menus of the menu bar. The most interesting one in this context is the "Cleanup" menu where you can move the current item to the trash (which can be undone from your desktop's trash can icon), delete it immediately (which cannot be undone), or lots of other actions that are more or less destructive.
There are also non-destructive actions to help you inspect the item in more
detail; for example "Check File Type" which starts the file command for this
item and shows you the result. Or you can start a file manager in that
directory, or a terminal window.
You can configure those cleanup actions. Use menu "Edit" -> "Configure" -> Tab "Cleanup Actions".
Select a treemap tile and use your mouse wheel to zoom in and out again: Each click on the wheel zooms the treemap one directory level further in or out.
Alternatively, use Ctrl++ and Ctrl-- like in all modern Internet
browsers, and Ctrl+0 (digit zero) to reset the zoom. You can also use the
toolbar actions with the magnifying glass icons.
Clicking on a treemap tile selects it, and it gets a red selection frame. Use
Ctrl-Click to add or remove one more tile to the selection; you can start
an action for all of them at once. The other selected tiles get a yellow
rectangle marker.
If you right-click a treemap tile, that will always select it first and then
open the context menu. If you have a multi-selection, make sure the mouse
cursor is actually on the last selected tile, or hold the Ctrl button before
opening the context menu, otherwise the previous selection may be gone.
In a deeply nested directory tree, it's sometimes interesting to see in the treemap what belongs to the directory of the currently selected item, and what does not.
Use the middle mouse button to select, and you will see white rectangles around the current item's parent, grandparent etc.; up to almost the top level. Everything outside that tree branch will be dimmed out a bit.
Click outside the current branch (in the dimmed-out area) to get it back to normal without white frames and without anything dimmed out.
You can use the middle mouse click pretty much like a left click in the
treemap; use Ctrl to extend the selection.
Drag on the divider between the tree view and the treemap to give either one
more or less space. Your last settings are stored in the QDirStat main configuration
file at ~/.config/QDirStat/QDirStat.conf.
Sometimes it's useful to get rid of the treemap momentarily to have more screen
space for the tree view. Use the F9 function key to quickly toggle it off or
on.
Some (few) users wanted to use only the treemap and get rid of the tree view completely.
This is technically possible (just drag the divider between the two views all the way up), and sometimes it may be useful to see a particularly aesthetically pleasing treemap in a better resolution and with less distracting GUI elements.
But it is strongly advised not to do this for normal work: The primary view is the tree view. It and the "Details" panel on the right (which would be gone as well) show all the detailed information that QDirStat is all about.
The treemap is intended as a secondary view which is no doubt very useful, but it has its limitations. For example, you only see the full path and the size of the currently selected item there, that's it.
If anybody finds that sufficiently useful to work with QDirStat on that basis alone, that is their choice, but any complaints about missing information or functionality that are now no longer accessible will be rejected.
In a very large directory tree with many (10,000 or 100,000 or more) files, the scaling will make some treemap tiles very small; a 4k file doesn't get much visibility in a 1.2 GB home directory.
QDirStat's treemap layout algorithm "cuts off" treemap tiles below a certain
pixel size: MinTileSize in the config file, by default 3 pixels. There is not
anything useful to render in a tile that small; it would only become pixel
mush.
So some directories contain a partly grey area to their bottom right: That is where those tiny tiles would go, but it would be useless to render them; they are too small to be identified or to be clicked.
This is not a graphical error, it's a feature to boost performance, and to prevent you from accidentially clicking on something that you have no way of knowing what it is.
The information that such a grey area gives you is that there is a lot of little stuff that you can't see on that zoom level, so zoom in if you are really interested.
To make this a bit more deterministic, select any other tile on that directory
level, then use the "Up one level" action (Icon: Arrow up; key: Alt+Cursor Up) to select the parent directory, then zoom in all the way (mouse wheel or
Ctrl++).
If you see any very large tiles, click on them and check what they are. Watch the "Details" panel. If you are in a system directory, it will check if it's probably a system file, and what installed software package it belongs to, if any.
If it's in your home directory, you might rediscover that CoolLinux.iso installation ISO image that you downloaded some time ago, then forgot about it.
The nice thing about this is that it doesn't matter at all how many levels deep in the directory hierarchy it's hiding away: You can see it instantly without opening any branches in the tree view. That one click on the fat treemap tile will do that for you, and then you can decide if you want to keep it, get rid of it, move it to a better location, compress it or whatever.
We've all been there... You click on something in your Internet browser, and it starts to download something; a movie or whatever. OK, so let's watch it. But... where did it go? Why doesn't the browser ask me where to store it?
Solution: Start QDirstat and select your home directory. Chances are that now you have big fat bright green blob somewhere deep in some obscure path below your home directory. That's your new movie; open a file manager there and move it to a more suitable location so you have realistic chance to find it again.
In these modern days, you need several different Internet browsers for different tasks; while one web app only wants to work with Firefox, another one insists on Chrome / Chromium. And if your favorite browser is Opera, you have to cope with three of them. Ah yes, and Thunderbird as a mail client which is also just another incarnation of Mozilla, just like Firefox.
And every single one of them insisits on creating its own cache directories,
and every single one of them consumes at least 50 MB, sometimes 500 MB. And
none of them is content with only one cache directory: They have several
caches for web pages, for images, for Javascript code, for their startup, for
thumbnails. And they are spread all over the place; not only in ~/.cache/
which is designed for that exact purpose, no, they also pollute ~/.config/,
and sometimes directories directly below your home directory.
It would be quite okay if their "clear caches" command would actually empty them all, but none of them does that; they empty some of them, but never all.
That makes backing up the home directory a royal PITA: You'll never see what
your carefully crafted rsync command actually does; its output will always be
polluted by a gazillion lines about all the cruft in those cache directories.
What you can do is to start collecting bookmarks of such directories and then go to each one and clear its content. To bookmark the current path, click the star icon to the left of the path (the breadcrumbs widget) just below the tool bar.
You can identify cache directories by a name containing "Cache", and by the visual appearance in the treemap: If you found a very large area with a large number (1000 or more) of grey tiles (e.g. they cannot be identified what they are) of similar size that belong together (which the white borders after a middle-click will show you), you may have spotted a cache directory.
It looks like you can actually spot a cache directory from outer space (ok, from a near earth orbit) with a treemap.
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SequoiaView @ TU Eindhoven, NL (for Windows)
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PDF Download "Cushioned Treemaps" paper by van Wijk, van de Wetering @ TU Eindhoven, NL from 10/1999
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PDF Download "Squarified Treemaps" paper by Bruls, Huizing, van Wijk @ TU Eindhoven, NL