Right—they fell sharply from 1945 to 1955 (it looks the data points are sampled every 10 years) but have been pretty stable in the post-war era, as I said. Note that OP stated the top tax rate was lowered starting in the 1970s.
For the richest tax is optional to the point where that get loans against their shares rather than pay capital gains tax. It’s an absolute disgrace and it’s got to a point where governments need to taxing them properly.