Unclassed choropleth maps

This map shows lightning strike density per square kilometer within the continental United States for the years 1996-2005. An unclassed choropleth is designed to show statistical data in a multi-colored value moving from smallest to largest, using lighter colors to portray smaller values and using progressively darker shades to portray progressively greater values.

Image Source: http://www.weather.gov/os/lightning/images/Vaisala_96-05_Flash_Map.gif