A great example of “lies, damned lies, and statistics”.
Graphs like this are useless … actually, they are worse than useless—they are intentionally deceptive. The “top tier” varies widely over the years. At points in our history, the top tier has covered only the Buffets and Gates of the era; now, it covers your doctor and lawyer and boss. Any useful graph of this nature would define its terms and adjust for inflation. So much of the blogosphere is filled with stuff like this—stuff that looks like information, but isn’t.
UPDATE: jakke beat me to this, and said it better: ”In 2010 dollars, in 1960 the top marginal tax rate was on income levels well over $3 million, whereas now the top marginal tax rate kicks in around $370,000.”