Bivariate Choropleth Map

 http://proceedings.esri.com/library/userconf/proc99/proceed/papers/pap171/p171.htm

Bivariate Choropleth Maps display 2 variables on a single map by combining 2 different types of graphic representations. The map above shows 2 seperate variables, graduated circle (point variables) to represent median house values and color shading to represent the population, all conjoined into the same map. This makes comparing variables very easy and interpretable.