Isopleths

An isopleth  is a contour line of equal or constant values. Lines of equal value are drawn such that all values on one side are higher than the "isoline" value and all values on the other side are lower. They are often used for population density.This map shows changes in the amount of acid rain resulting from legislative efforts of the Clean Air Act Amendments.

http://www.dep.state.pa.us/dep/deputate/airwaste/aq/acidrain/isomaps.htm