The colors indicate the marginal tax rate: black for low, red in the middle, and yellow for high. The horizontal axis is the tax year, and the vertical represents taxable income, log-scale, normalized to 2010 dollars with the Bureau Of Labor Statistics’ monthly CPI-U figures. The bracket data comes from The Tax Foundation and the IRS, and the effects of Social Security, capital gains, AMT, and other tax varieties are not included.
Through most of the century, brackets were much closer to a continuous scale. There was a big shift in thought though in the 1980s, when Ronald Reagan was elected president. The brackets became much more distinct. The idea has more or less stuck over the past two decades.
Of course what sticks out the most is the 90% income tax during the mid-1900s. Earn $10 million. Give the man $9 million of it. That seems sort of, uh, wrong. The range between lowest and highest is also really big at 70 percentage points. It's only a small difference of 15 percentage points nowadays. Much better.
Update: As noted in the comments, my knowledge of tax brackets is amazing. I should be a CPA. Here's the corrected math. The amount you earned over $10 million in 1950 is what would get taxed 90%. So if you earned $11 million, $900,000 of the last million would go to the man. Subsequently, the first $20,000 would be taxed 20%, then the next lump 30%, so on and so forth. Thanks, all.