In case you wanted to see a version which adjusts transparency by population density:
Image Link
Any chance of this same map but with swing?
Does show that population density in areas west of Minnesota/Dakotas border and west of Dallas all the to West Coast are quite low. Yes you have some populated states, but most concentrated in cities and much of the rest of the Plains and Mountain West is largely empty. In fact that is the one part of the United States where most counties have population densities lower than Canada and Australia (off course in both of those two population heavily concentrated near the US border in former and ocean in latter). Areas east of Mississippi much more dense and population densities they are somewhat more comparable to Europe although only in Mid-Atlantic and Southern New England states do they get quite that dense.
