Bivariate Chloropleth

source: http://rkpjrhist615.wordpress.com/category/geography-310/

Bivariate Chloropleth maps display two variables (bi-two:variate-variable) on one map by combining two different methods of depiction (usually colors or symbols). In the case of the example map above, rural population in Washington state counties can be visualized both as a share of county populations and as direct rural population size. Where you see large, dark blue circles in light green counties, you can assume that though the rural population is relatively large its share of the overall population is small (these counties probably have large overall populations). Conversely, small light blue circles in dark green counties signal small, mostly rural overall populations.