It was around the 70s that the key shift likely happened. This is where productivity (I think measured in money) continued to grow while wages stagnated. So showing some time before this would be beneficial.
The chart also only plots union membership from 1973. Given the US’ history of unions, I think it would be far more interesting to go back to 1900, in particular to track membership numbers across events like the Battle of Blair Mountain 1921.
In general this graph seems to show a nice correlation but really doesn’t dig in enough to say anything meaningful.