Bivariate Choropleth Map

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

Bivariate choropleth are maps that display two variable on a single map. Those variable may be identified by different colors or symbols. In the map above, the two variable shown are population density in 1990 and population change from 1990 to 2000.