October 16, 2006. I fixed a minor bug in the axis labels of the
R version of mosaicplot(). The revised code is available
or you can wait until the next release of R.
My original code has now been included in S-Plus and R. In S-Plus, I think it is part of library(MASS). In R, it is in the main distribution, function mosaicplot(). Most recently, I've integrated mosaicplot() with pairs() and boxplot() to create a more generally useful pairs plot.
For references about mosaic plots, see Emerson (1998), "Mosaic Displays in S-PLUS: A General Implementation and a Case Study." Statistical Computing and Graphics Newsletter, Vol. 9, No. 1, 1998.
Nov 8, 2005: I just learned of a new implementation of mosaicplot, in package vcd: Visualizing Categorical Data by David Meyer, Achim Zeileis, and Kurt Hornik. I also believe that Kurt Hornik is the person responsible for adapting my mosaic implementation for R, and for that I offer a belated thanks!