Put together by Randall Munroe of XKCD, the visualization relies on the fact that California is long and thin. He's taken monthly data recorded over the past 14 years, squished the 2D maps down to 1D columns—providing a simple but representative rendering of drought across the state's length—and then stacked them next to each other to show how it varies with time. Essentially, he's used California as its own graph axis.
It's a neat data trick, but the chart itself is startling. Let's hope that black patch starts shrinking real soon. [XKCD]