Choropleth

Choropleth [ˈkɔːrəˌplɛθ] : A map with geographical areas colored in relation to a data variable.

It is used to display patterns or variation of the variable in different districts of a region. For example it’s popular to show different voting behaviour between the electoral districts of a country.

The type of data determines what color scale to pick for the map. Sequential data is often displayed with a color gradient from light to dark. Maps showing diverging data, for example the derivation from a specific comparative value, get a scale with two color gradients and a fixed color for the comparative value. If your data is qualitative, every category gets its own individual color in the map. A great tool to find a good color scale matching your data is colorbrewer.