This is a choropleth map that provides a rough estimate of taxes per capita. The Internal Revenue Service provides historical data on total income taxes paid by county. The per capita estimates come from dividing these values by total population as reported by the 2010 census.
Two caveats: The data for taxes is not the same year as the data for population, and per capita is for the aggregate population rather than the working population.
For the former, migration between counties isn’t so rapid that it would drastically affect the results of the map. For the latter, average family size might be a factor in the calculation—as larger family sizes per household would bring the tax per capita value down—but this does not appear to be too big of a factor on the surface. However, more research is required to know for sure.
