Proportional circle map

http://personal.frostburg.edu/sbriggs0/maps.htm

Proportional circle maps display data in relation to the size of the circle.  Proportional dot maps are an effective way to visualize variables or things subject to change. They are often used to portray different populations in certain areas. The map shown above represents the number of Walmarts in each state in America. As you can see by the legend, the more Walmarts there are, the bigger the circle is.