That has to be the worst x-axis on a graph I've seen in a long time. The grid lines measure 0-10th, 20-30th, 40-50th, 60-70th, 80-90th, 95-99th, 99.99th, Top 400? That's atrocious, not only is it non-linear, not only does it have a line with regularly spaced dots giving the impression of continuity, but it skips from percent groups to absolute numbers? If we're talking about historical taxes why are we looking at tax…
I don't understand your complaint, isn't average tax rate a percentage of income? https://en.m.wikipedia.org/wiki/Tax_rate
The common argument is that the 70% rate of the 1950's was never actually paid by anybody and the effective tax rate was much lower. By looking at actual taxes paid that entire argument can be skipped.
Anybody who studies historical taxes professionally should know this, so when they use a worse measure it looks like they're doing so because it fits their narrative.