A choropleth map is a thematic map that uses colored or shaded areas to display statistical variables such as population density or per-capita income. As the name suggests, color is the important part of a choropleth map. A color scale is assigned to categorical or numerical data, and the value for each region is used to color the region.
The earliest choropleth map is credited to Charles Dupin, a French mathematician who, in 1826, published a thematic map using shadings to show the distribution of illiteracy in France.
Choropleth maps are based on predefined political borders such as states or counties. The color provides an easy way to visualize the distribution of a measurement across a region. However, there can be perceptual issues. For example, in the United States, the Southwest takes up a large area on a map but the population is relatively low, while New England takes up a very small area on a map but has very high population numbers. For population-related information, then, New England can appear under-represented.
When using a choropleth map, it’s important to use an appropriate color scale, so the map’s color legend should use a progression that mimics the data. For categorical data, five to seven color categories are generally the most that should be used, so that the viewer can easily identify the color and match it with the category.