Range graded proportional circle map

This map shows EID events throughout the world. We can see that the majority of them occur in the U.S, and most of the occurrences in the U.S happen in the Northeast. In range graded maps, it is all about symbol size. The data are divided into categories, where the map reader can easily distinguish one class from the other.

http://www.nature.com/nature/journal/v451/n7181/images/nature06536-f2.2.jpg