An Easy Way to Make a Treemap | FlowingData
http://flowingdata.com/2010/02/11/an-easy-way-to-make-a-treemap/

library(portfolio)

I think this would be pretty easy to do with gpplot2, but Portfolio looks like its worth checking out too.

Here's a really easy way to make your own treemap in just a couple lines of code. We're looking to make something like this: