Univariate Choropleth Map

http://www.colocarto.com/custom-thematic-maps.html

A univariate choropleth map displays data from only one variable. Though still following satisfying all the criteria to be considered a choropleth map, this type of map is much simpler than other choropleth maps that displays data of multiple variables. In this example, the estimated median household income for 2008 is the only variable analyzed and shown on the map. The scale uses progressively darker shades of one color is used to show an increase, and make it easier for viewers to compare different regions of the U.S.