Proportional Circle Maps

http://www.geog.ucsb.edu/~jeff/gis/proportional_symbols.html

A Proportional Circle Map  uses circles to represent a total data value within a geographical area on a map. Circles are used over other shapes because statistical information can be more accurately scaled to size on a map using the radius of a circle to represent an area. The bigger the circle, the bigger the geographical location the circle represents. To the human eye, circle proportion is an easy way to understand and differentiate scaled values on a map.