Teaching:TUW - UE InfoVis WS 2008/09 - Gruppe 06 - Aufgabe 1 - Treemap

Figure 1: Tree as directed graph [Shneiderman, 1992]

Figure 2: Same tree as Treemap [Shneiderman, 1992]

Figure 3: Colour-coded Treemap [Shneiderman, 1992]

Figure 4: Treemap for Images: PhotoMesa [Bederson, 2002]

Figure 5: Treemap of the Usenet [Smith, 2001]

Treemaps are a 2D space-filling visualisation method for hierarchical, tree like structures.
They were introduced by Shneiderman [1992].

Treemaps aim at visualising a certain attribute of hierarchically structured data in a 2D space, for example the size of files in various levels of folder hierarchies on a hard-disk (which was the motivation for developing Tremaps in Shneiderman [1992]). Then traditional ways of visualising hierarchical structures as (directed) graphs, as in Figure 1, can show relations between the nodes, but fail to visualize attributes such as size or importance of nodes. Treemaps try to overcome this by visualising each node as a small rectangle in the 2D space, as in Figure 2.

In detail, the Treemap algorithm has the following properties

• Input: a root node, and a 2D space.
• The number of outgoings edge from the root node determines the number of partitions in the space.
• The sum of weights assigned to each sub-tree determines the size of the partitions.
• Then, recursively the space assigned to each sub-tree will be partitioned.
• The sub-tree are partitioned alternatively horizontally and vertically, with odd levels being horizontally, and even vertically.
• Additional attributes can be visualised with a colour coding (e.g. file-types as in Figure 3).

This algorithm was named "slice and dice".

The Treemap method has been well studied, and a wealth of variations of the original algorithms have been published (compare [Shneiderman 1998]), among them:

• Squarified Treemaps try to render the shapes of nodes rather as squares than as (long and narrow) rectangles, which are difficult to compare. To achieve this, the order of the nodes is disregarded. Squarified Treemaps may lead to stability issues when node sizes change, causing the order of nodes to potentially change dramatically ([Benderson 2002]).
• An attempt to overcome the stability issue is presented in [Shneiderman 2001] by balancing order and visual appeal of squared slices.

Applications of Treemaps include:

• Visualisation of File-system usage [Shneiderman 1992]
• A photo image browser, where the issue of having to display fixed-size content (the images) in shapes with potentially very differently aspect ratios [Benderson 2002]
• A visualisation of the newsgroups in Usenet, and the frequency of specific types of newsgroups [Smith 2001]

References

• [Shneiderman, 1992] Ben Shneiderman. Tree visualization with tree-maps: A 2-D space-filling approach. ACM Transactions on Graphics, 11(1):92-99, 1992.
• [Shneiderman, 1998] Ben Shneiderman. Treemaps for space-constrained visualization of hierarchies. Created: Dec. 26th, 1998, Last retrieved Nov. 5th, 2008. http://www.cs.umd.edu/hcil/treemap-history/
• [Shneiderman, 2001] Ben Shneiderman, Martin Wattenberg. Ordered treemap layouts. In Proceedings of the IEEE Symposium on Information Visualization 2001 (INFOVIS 2001), pp 73 - 78.
• [Benderson, 2002] Benjamin B. Bederson, Ben Shneiderman, B. and Martin Wattenberg. Ordered and quantum treemaps: Making effective use of 2D space to display hierarchies. ACM Transactions on graphics
• [Smith 2001] Marc Smith and Andrew Fiore. Visualization components for persistent conversations. In Proceedings of the Conference on Human Factors in Computing Systems, pp 136-143. Seattle, Washington, United States, 2001