Univariate Choropleth map

A univariate choropleth map is one that displays a single variable or attribute over a large area, and its magnitude.  The data is displayed generally by a similar color scheme.  This map shows the distribution of hay throughout counties in the United States.  The counties with the most hay are shaded a dark green, with counties producing less hay outlined in light green.
http://www.nass.usda.gov/