Proportional circle map

 Proportional circle maps are statistical maps in which there is a constant variable, such as the amount of  corn produced by the U.S. in a given year.  Formulas are used to control the area of the circles which will accurately reflect some attribute value. When creating proportional circle maps a vector graphics program is used. This map shows population in U.S. capitals.

Link: http://sohosp.blogspot.com/2008_12_01_archive.html