Univariate Chorpleth map

http://www.agcensus.usda.gov/Publications/2002/Ag_Atlas_Maps/Crops_and_Plants/index.asp
Univariate Chorpleth maps are maps that unlike Bivariate maps, strictly display one measurable variable. These maps are simpler and used when showing one phenomenon.  The map above shows the acres of harvested corn for grain across the U.S. The different shades show the different amounts of acres harvested.