Range Graded Proportional Circle Map

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

A range graded proportional circle map is a type of symbol map that utilizes a circle size within a certain range of data. Point data is mapped with a circle instead of a dot, and there are a finite or set number of sizes used for the circles. A range graded proportional circle map is used when the the map makers want to show an average to fit with a set number of circles. The map shown above is an example of a range graded proportional circle map of internet users in 2004 in western Europe.