Unclassed Chloropleth Map- Lighning Strikes

The above map is an example of an Unclassed Chloropleth Map.  Unclassed chloropleth maps are a good visualization technique to portray statistical data.  This map shows the frequency of lightning strikes in the United States.  Maps of this type represent data through changing shade of color.  In this case, the places with most frequent lightning strikes are pink and orange.
http://www.weather.gov/os/lightning/images/Vaisala_96-05_Flash_Map.gif