Isopleths

http://geographyfieldwork.com/DataPresentationMappingTechniques.htm

Isopleth maps are much like choropleth map, however, the data is not grouped in a predefined unit. When comparing data change over time, boundary line found in choropleth maps produce abrupt changes. Isopleth maps synchronize the data presented at all points to show a uniform change such as the map above that depicts precipitation.