Imagine a pie chart where each slice projects a set of subslices.
The SunBurst visualization project at Georgia Tech, led by John Stasko http://www.cc.gatech.edu/gvu/ii/sunburst/
There's also an implementation in SVG in Mozilla over at [SVGgraphs.mozdev]. The circular TreeMap inspired graph worked in earlier versions of Mozilla, but has fallen to XBL changes. --- AndyEdmonds