Adding on to my own analysis about time series trends in income tax progressivity, Professor Munger offers this analysis from the Tax Policy Center.

• In 2012, the top quintile of the income distribution received 52.5 percent of income and paid 68.3 percent of federal taxes.
  Source: Tax Policy Center Table T12-0200.

This series includes payroll and other taxes that weren’t included in my analysis. It would be nice to get a time series on this as well.