Treemaps are a type of visualisation created in the early 90s by Ben Shneiderman, that allow you to represent hierarchies in a way that optimises the space filling and, moreover, enables you to see their attributes and to identify specific patterns or properties of the hierarchy using colours.

The problem that Ben Shneiderman was trying to solve in 1990 (see his page on treemaps) was that of representing the directory hierarchy of a hard disk full of data pertaining to several users, in a compact and effective way. Finding big files to erase or discovering the way the different users were using their space was a major issue.

Hierarchies can be represented in several ways, for example by using Venn diagrams or tree structures (as the Windows explorer does), that occupy a lot of space and don't indicate in a compact and effective way the properties or the "branches" and "leaves" of the tree.

Click on the image to zoom it.	Click on the image to zoom it
Representation of a hard disk's directory hierarchy as a tree. (Windows 98 Explorer on my hard disk)	Representation of the directory hierarcy in an early version of TreeViz. (from "Treemaps for space-constrained visualization of hierarchies" by Ben Shneiderman)

In that situation Shneiderman decided to convert the classical tree into a flat representation filling all the available space. In it each file becomes a rectangle of area proportional to its size. The rectangles are nested one inside the other as a function of the level of the hierarchy we are descending to, alternating the portrait and landscape orientation.

In fact the area of a treemap can be made proportional to whatever magnitude we find interesting, which gives part of its versatility. On the other hand, the nodes and leaves of the tree can be coloured allocating a different meaning to each colour.

For example, in the visualisation of my hard disk, a big, vertical rectangle includes all the files of the Windows directory (see the figure in the HTML version above mentioned). A non-negligible part is the swap area, which is represented as a white rectangle in the lower part of the rectangle corresponding to Windows.

Click on the image to zoom it.	Click on the image to zoom it.
The "Classical" Treemap of a hard disk. The biggest, bluish part is free space. The yellow rectangle is the Windows directory. The swap area is the white horizontal area to the lower left.	The same Treemap, obtained with the "Cushion Treemap" technique of SequoiaView. Notice the change that  the 3D effect produces, compared with the classical treemap.

The colours of the rectangles are allocated according to the file extensions. However this doesn’t stop you finding important black areas corresponding to small files with unusual extensions, like .s3d, .qtx, etc.

One of the first versions developed by Shneiderman’s team, called TreeViz can be freely downloaded (Macintosh only).  More recent versions (Treemap 2000 and 3.0) aren’t available for free although a downloadable demo do exist. For the Windows platform there’s a recent implementation called SequoiaView, made by the University of Eindhoven in the Nederlands that extends the treemap concept by representing the rectangles as 3D cushions. Their creator appropriately calls it "Cushion Treemap". It’s amazing the clarifying result of applying this 3D effect. It’s worth downloading it and playing with it in order to get an idea of the possibilities of this technique.

Since the beginning in the 90s the Treemap concept has been evolving, being commercialised with the name of DiscMapper  by Micrologic Corp that owns a patent (US Patent # 5,987,469). 

TreeViz, SequoiaView and DiscMapper use the Treemap concept to represent the files of a hard disk. Nevertheless, the Treemap potential goes beyond this limited scope, as we’ll see in the next issue.

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