Bivariate choropleth maps

A bivariate map displays two variables on the same map. A bivariate map includes line features, polygon features, and point features. These maps are used when there is two variables in question that need to be mapped. The bivariate map below shows  the population  density in Russian in 2002 and how it has changed since 1989.

Link: http://www.geog.le.ac.uk/russianheartland/DemographicMaps/Raions.html