My graph also shows median income, and it clearly shows it has not stagnated, but risen, and not by any small amount.
There’s a story to tell here, but you’re not showing it.
Roughly, the way it "should" work is that if Company A and Company B are in the same industry, and Company A is more productive than Company B, free market forces cause Company A to pay more than Company B, thus improving their ability to compete for workers. Generalize this to an entire industry of competing companies, and then to an entire economy.
Refuting the (willful) abuse of statistics is like trying to refute numerology with reason.