One ugly chart shows how Bill Ackman's investments have been decimated

Hedge fund titan Bill Ackman's Pershing Square Holdings, the fund's publicly-traded vehicle, suffered its worst year in history, falling 20.5% in 2015.

It was a year Ackman won't "forget," according to the fund's fourth quarter investor letter.

The fund was dragged down by Canadian drugmaker Valeant Pharmaceuticals. The stock's massive decline resulted from scrutiny in Washington over drug price increases and accusations from a short-seller. Valeant has denied any wrongdoing.

It wasn't just Valeant, though. All of Ackman's long stock positions got brutalized in 2015, and they're continuing to get clobbered in 2016. Pershing Square Holdings was last down 14.5% through January 19.

Ackman, a well-known activist investor, makes large, concentrated bets in a handful companies.

"While our portfolio is highly concentrated, we are highly diversified in the industries in which we invest: sweet snacks and chewing gum, industrial gases, real estate, specialty pharmaceuticals, specialty chemicals, frozen foods, animal health, housing finance, railroading, and quick service restaurants. One would not expect a substantially greater than market value decline in our portfolio due to recent company-specific and macro events as has occurred since August," Ackman wrote.

The fund's five largest holdings include Valeant Pharmaceuticals, Air Products & Chemicals, Canadian Pacific, Mondelez, and Zoetis.

Ackman is also an investor in Platform Specialty Products, which has dropped 42% this year. Meanwhile, Howard Hughes Corp, another one of his long equity holdings, has fallen about 16%.

Here's a chart showing the performance of those positions.

Ackman's only short is Herbalife, a multi-level marketing company that sells weight loss shakes. Ackman has been crusading against Herbalife since December 2012 because he believes the company is a "pyramid scheme." He's betting that the stock goes to $0.

Shares of Herbalife have fallen more than 14% this year.