We can't conclude from this chart that the two curves are independent - only that they have been independent historically. One explanation that leaps to mind is

We can't conclude from this chart that the two curves are independent - only that they have been independent historically. One explanation that leaps to mind is that the total tax level will remain constant because the government will always tax sufficiently to fund its programs, and those have stayed roughly constant as a proportion of GDP. This leads to a seeming noncorrelation between any segment of the tax code and total tax revenue. Even if the above explanation is far off-base, I hope it shows how a historical noncorrelation doesn't necessarily mean a lack of causal connection.