An isotherm is a line of equal or constant temperature on a graph, plot, or map; an isopleth of temperature. For example, isotherms are commonly seen on weather maps to show large-scale temperature distributions.
Drawing Contour Plots (http://avc.comm.nsdlib.org/cgi-bin/wiki_print.pl?Drawing_Contour_Plots). A lesson plan that deals with drawing various isopleths including isotherms.
... The per capita income for the town is $29,939. 5.5% of the population and 2.2% of families are below the poverty line. Out of the total people living in poverty, 5.5% ...