(I know it's hard to read--if you want to look at the clearer version, click here.)
It shows that since Harry Truman, every Republican has left office with a lower tax burden as a % of GDP than when he took office, and every Democrat just the opposite.
The most optimistic view: Republican policies keep taxes low and spur the economy. Another view: Republicans have enough understanding of economic cycles to make tax policies that do not outpace the growth of the economy. An unlikely view: total luck.
It's easy to imagine a coincidence explaining this phenomenon over a short amount of time, but after 60 years, I think we are establishing a pattern.