Bivariate Choropleth map- Purple America

A bivariate choropleth map compares two different variables of data as one visualization.  The two sets are generally differentiated by color.  The image above is known as Purple America.  It shows the voting results of the 2004 election divided between Democrats and Republicans.  The purple areas represent places where the political preferences are very diverse, and there is not a strong presence of one or the other, but rather equal distribution.
http://www.princeton.edu/~rvdb/JAVA/election2004/purple_america_2004.gif