Any tree structure (hierarchy) can be visualized as a tree
but it could also be visualized as circles embedded within one another (like a Venn diagram)
Here's a description of a program which creates diagrams of embedded circles to represent hierarchies:
http://web.media.mit.edu/~dc/research/fashion/visualizing.html
(the authors seem to be David Ta-gang Chiou and maybe also Kan Liu and Professor JudithDonath)
Because you can't tile a circle completely with other circles, this kind of tree diagram wastes space. The more levels of hierarchy, the more waste. (The math is straightforward, it gets worse exponentially.) A natural way to fix the problem is to use rectangles instead of circles. This idea leads to a TreeMap.
BrokenLinks to both diagrams and also the description (15 July 2006).