PROPORTIONAL CIRCLE MAP

https://www.agronomy.org/publications/aj/articles/102/4/1158
Proportional circle maps are used to represent particular information or values with the use of circles. The bigger circles represent larger values, while the smaller circles represent smaller values or data.  This is an example of a proportional circle map, showing switch grass biomass yields across the United States, predicted by the empirical model. The black circles show the observed upland yield, and the red circles show the lowland yield of switch grass.