This was posted here months ago, and shown to be grossly inadequate then. It's entirely meaningless because you could plot anything on that graph and draw an identical conclusion. Surely there must be something that does fluctuate and effect the tax revenue as a percentage of GDP, but if you plotted it on a graph it would appear not to.

Moreover, top marginal tax rate is an all but irrelevant metric since the ultra-wealthy pay considerably lower due to capital gains.