Cool data source. Thanks for sharing. In case anyone is curious about the early history of the income tax, or why it is so spotty prior to 1913:

The income tax was first imposed in 1861 to help pay for the Civil War. Prior to that, the government got most of its income from excise taxes and tariffs.

In 1895, the Supreme Court declared the income tax unconstitutional, since it was not apportioned fairly between states, which is why the government added the 16th amendment in 1913, which gave it the power to impose an unapportioned income tax.