One of the big stories in US history is the creation of a nation out of a diverse group of sections—particularly by the convergence of the South on the rest of the country. We know this, at some rough level; the South was rich in the era of slavery1 then poor after the Civil War and then in the middle twentieth century began to look more like the rest of the country.
So these are arranged by state, and color-coded by region. You can see the S shape get shallower, and you can see the South creep up and become better-distributed through the curve, I think.
The figures here come from Kris James Mitchener and Ian W. McLean’s work on “The Productivity of US States since 1880”, Journal of Economic Growth 8 (2003): 73-114. Mitchener and McLean have price indices for each state, to create figures that are comparable across states for price-adjusted income per worker. To render the figures comparable over time, I divided each by the mean for the given year. So you’re seeing the degree of divergence represented here, not actual dollar figures for earnings.
1The extent to which this was true of course should remind us that the wealth of a region may mean the wealth of only a few people in that region.