maximum threshold values and its height calculated such that the box area is proportional to the number of districts included, then colored with the map symbol used for that class. Alternatively, the histogram may be divided into a large number of bars, such that each class includes one or more bars, symbolized according to its symbol in the map. This form of legend shows not only the threshold values for each class, but gives some context for the source of those values, especially for endogenous classification rules that are based on the frequency distribution, such as quantiles. However, they are not currently supported in GIS and mapping software, and must typically be constructed manually. See also Cartograms, which are often colored as choropleths. Chorochromatic map Dasymetric map Dot distribution map Heat map MacChoro Proportional symbol map References Further reading External links ColorBrewer – color advice for cartography A choropleth map generator for the US Thematic Maps Statistical charts and diagrams Color scales