Continuously Variable Proportional Circle Map

http://geographyfieldwork.com/DataPresentationMappingTechniques.htm

Above is a Continuously Variable Proportional Circle Map, these map types use shaded areas according to a key to represent values numerically. Population density is being expressed in this particular map, this type is closely related to a choropleth map, which are useful for indicating differences to be expressed through a map image. The map above is a report completed by the Czech Republic that is meant to represent population density.