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 (link appears to be broken 15Sept05)
(the authors seem to be David Ta-gang Chiou and maybe also Kan Liu and Professor Judith Donath)