This chart is a ticking time bomb for stocks

The last six years for the U.S. stock market have been an indomitable buying orgy fueled by the Federal Reserve's zero interest rate policy, causing the S&P 500 to triple in value. One metric says it may have gone too far.

Borrowing to buy stocks, as measured by the NYSE monthly margin debt figures, ended 2014 at its highest level relative to the size of the U.S. economy since 1929. In absolute terms, it's the highest ever.

"If this doesn't count, then we might as well throw away everything we've ever learned about risk because there is none," said Alan Newman, who sent this data to customers of his Crosscurrents newsletter last week. "The manias of 2000 and 2007 now both look like small potatoes by comparison. This is total insanity."