Your graph clearly shows that Reagan DID help cause it. The top marginal tax rate was 70% when Reagan took office, same as it was 15 years earlier, then Reagan successfully got through 2 massive cuts and it was 28% when he left office.
There was also a huge drop in the mid-60s and Reagan didnāt cause what happened in France or Germany. I couldnāt find a chart with more countries than that over that time period, but I expect it was quite widespread. Reagan was part of it happening for sure, but he didnāt start it, and, the President doesnāt unilaterally set the top marginal tax rate in the US. They arenāt even the party that is primarily responsible for it.