Bivariate choropleth map - population density and change in the U.S.

 https://www.e-education.psu.edu/geog486/l5_p5.html

 

The difficult part in displaying two different variables (bivariate) in one map is that the visualization must be clear – the viewer must be able to distinguish the two variables from one another and still see their relationship with the area.  In this bivariate choropleth map, hue is used to communicate the three classes of 1990 population density by county and value (intensity of the hue) is used to communicate three classes of percent change in population from 1990 to 2000.  It looks like most of the population increase in high-density areas occurred along the coast.  Most of the population increase in low-density areas seems to have occurred throughout the West.