R is an open source software program that is extremely useful for data analysis and data visualization.
An updated HTML web-book version, PDF Version, and e-book version of my Introduction to R.
A Quick Introduction to ggplot2, a graphing library for R (HTML Web Book) (PDF) (e-book).
I have developed some code to use University of Michigan Colors on R graphs. This code is also available as an R package that you can download and install.
More R resources can be found on my blog.
For some sense of some of the kind of things one can do with R, you may wish to take a look at my Data Visualization or Statistical Visualizations pages.