Support tree graphs #42

Moved from Google Code issue #568.

Add support for tree graphs. This type of graph is a relative to bubble graphs that size rectangles depending on a data value. Additionally these rectangles are automatically clustered so that no free space remains between them. One example is here:

http://flowingdata.com/2010/11/03/review-atlas-of-science-visualizing-what-we-know/

A very cool application is SequoiaView developed by the University Eindhoven in 2002 which shows disk content (files) in a tree chart:

http://w3.win.tue.nl/nl/onderzoek/onderzoek_informatica/visualization/sequoiaview/