Bivariate Choropleth Maps

http://www.personal.psu.edu/faculty/c/a/cab38/GEOG321/14_multivariate02/multivariate.html

A bivariate choropleth map is used to display two variables on a single map by combining two different sets of colors and is very useful in graphically illustrating the relationship between two spatially distributed variables.  The bivariate choropleth map above measure percent change and the population density across the United States.