Bivariate Choropleth Map

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

A bivariate choropleth map is able to map two cartographic variables on the same map.  This is a map of the US and it is a bivariate choropleth map because it is displaying both the population density in 1990 as well as the percent change from 1990 to 2000.