Unclassed choropleth map

http://www.nass.usda.gov/research/atlas02/crops/Vegetables%20and%20Melons%20Harvested/Vegetables,%20Acres%20Harvested%20for%20Sale-choropleth%20map.gif

Unclassed choropleth maps show data in color shadings that are proportional to the data size.For example, with the map above the more acres of land with vegetables harvested for sale the bigger area and darker the color used to denote that.