Bivariate Choropleth Map

http://www.cartogrammar.com/blog/indiemapper-is-here/

This is an example of a bivariate choropleth map because it represents two different data amount simultaneously. This map is representing different age populations in the US back in 1900. If you are looking at the key you will see that the top axis is ages 5-17 and the right axis is 50-64. It then can use different color schemes that will represent the data points that were being used.