Bivariate maps are maps that display two varibles in the same mapping field. This is commonly seen in choropleth maps, where different colors can easily distinguish the different variables. This example looks at several commonly mapped variables in mapping that have a strong correlation. Because of this correlation, this form of mapping is an excellent venue to choose.
Found - www.worldpolicy.org
This blog is a map catalog in which I am displaying fifty different types of maps and a tid-bit of information that looks into the purpose of the map!