I mean, the section where the graph is contained is entitled "Risk of ground-water contamination by nitrate varies across the United States"

The risk of ground-water contamination by nitrate depends both on the nitrogen input to the land surface and the degree to which an aquifer is vulnerable to nitrate leaching and accumulation. Variables describing nitrogen input and aquifer vulnerability were estimated and compiled in a national map (figure 1) using procedures described by Nolan and others (1997).