The chart comes from James Lawrence Powell, a geologist, science-writer, and former professor, via DeSmogBlog. Powell reviewed 13,950 peer-reviewed scientific articles published between January 1991 and November 9, 2012 that mentioned "global warming" or "global climate change." The grand total of articles that questioned global warming or whether rising emissions are the cause: 24. That's 0.17 percent of all the literature on the topic.
Powell's review covers more papers than Naomi Oreskes' oft-cited 2004 study in Science