Isopleth

Isopleth maps use contour lines to join points of equal value across many variations.  This Isopleth map in particular is showing the PH levels and how sensitive each region is.  By looking at the legend, the dark blue areas are showing where extreme sensitivity took place.
http://www.globalchange.umich.edu/globalchange1/current/lectures/kling/water_nitro/water_and_nitrogen_cycles.htm