Did you read your own chart? Between 1942 and today, it ranges from ~14% to ~21%. I don't know if you're aware of this, but 7% of GDP is a big deal. Incidentally, the same chart you point to shows how tax income is about as low of a share of GDP as its ever been since 1950.
I've seen the same point made elsewhere, and the point isn't really that tax revenue stays constant as a % of GDP, it's that there seems to be a ceiling around 20%. The implication being that attempts to raise tax revenue greater than 20% of GDP are likely to fail.
And the people implying this are usually anti-government nuts.
The fact is tax increases of any kind are currently hard to get. Maybe as benefits for seniors start disappearing we'll see that change since seniors vote. If we ever get back to 20% of GDP for Federal tax revenues there's no special reasoning that says getting to 21% is going to be any harder then going from 18% to 19%. The whole 20% thing is just silly.
"Did you see the chart? That's what the empirical data shows."
It doesn't show any such thing. The whole truth is that from 1940 to 2000 average taxes paid including state and local went from 18% to 32%. Plenty of governments around the world and a number of US states are over 20% in federal taxes alone.
"Tax avoidance behavior increases as tax rates increase, you have moving targets."
This is obvious but, unfortunately, does not get you to "it's impossible to collect more then 20% federal taxes"
