Univariate Choropleth

source: http://wwwnc.cdc.gov/travel/yellowbook/2010/list/maps.aspx

A univariate choropleth maps display a single variable using colors to depict the varying degree of that variable. In the example above, the incidence of poverty is depicted in degrees of low (yellow) to high (dark brown). Using this type of map makes it convenient to isolate a variable of information and quickly determine its distribution over a geographic area.