I would estimate in 1600 there were trades like bricklayers or wheelwrights who earned multiples of the average income, but comparable work today, even with overtime, is average or less. It didn't get that way overnight, one generation gives way to another, as in the graph.
For example there were far different people enjoying the peaks to the right of Bill Clinton than there were suffering the downturns beforehand, which continued for so many. So the adjacent data there actually comes from two different places but it's the best generic income statistic we have.
Looks like the professor has done a realistic job in his own way of rationalizing some moving targets through history and coming up with some figures that may be about as comparable as they can be over the decades. I can only imagine it took a lot of math to visualize this as meaningfully as it is.
But remember it's the well-heeled people who bring the average up disproportionately more as the income disparity between high & low widens over the decades.
If you consider a time when a dollar had been an ounce of silver like forever (about the left half of the chart), then made it through the Nixon & Reagan Recessions, you would have seen how the average person (not the average income) was too devastated to participate in very much of the upticks since those crushing years.
But still subject to downticks being amplified to push them surely below the trough every time since. As the middle class, even the upper-middle class, had their wealth systematically removed by predatory forces that caused even more of a downward spiral in the average and below-average earners.
Often dipping well beyond the belt-tightening of the '80's, while the well-to-do maintained their compensation during the same adversity, not much differently than they're doing now by declaring higher dividends. Keeping the high-rollers in position to exclusively benefit when it does turn around, leaving everyone else further behind each time.
And in terms of "per Tax Unit", the left 2/3 of the graph is with one earner, but after that it's two different breadwinners for the most part.
Even though these are legitimate statistics, and look as similarly adjusted for inflation as other experts, it can be definitely confirmed that average income alone, no matter how well adjusted, is not a very accurate indicator of how the income of the non-exceptional population has been devastated by inflation worse than average. From what we know now, or at least the way it appears. If they could only have half of their dollars paid to them in half ounces of silver, they'd surely be a lot better off, but probably still not where it should have been, before all these additional means of devaluing the currency became possible.
Remember the way Bill Clinton was like when he started campaigning, it was the first contingent of overtime workers to have worked 20 years bringing in two incomes without getting ahead at all, compared to their fathers who had accomplished way more in their 20 years on one income.
And there were even more younger people who recognized that the promise of employment just wasn't what it used to be, since at the time it looked more hopeless than ever going into the future.
So one of his campaign promises was to grant $5000 to any American who would start their own business, employment hadn't been much hope for quite some time.
By design you were supposed to get an ounce of silver for each dollar you worked for.
That design changed long ago.