
Bivariate choropleth maps show more than one variable, like in this example it shows the amount of violent crimes, rapes, robberies and murders in the US in 2003 vs 2004
http://upload.wikimedia.org/wikipedia/commons/thumb/7/78/USA_states_population_density_map.PNG/800px-USA_states_population_density_map.PNG
http://ecrp.uiuc.edu/v8n2/birbili/images/fig2conceptmap.gif
http://www.cs4fn.org/fundamentals/images/ghostface.jpg