Randall Munroe has really outdone himself with today's xkcd. The infographic shows when a certain story was published and then compares that date to the length of time — either in the future or past — to when the story actually took or will take place.
To understand this chart, called "Stories of the Past and Future," you have to divide the top portion — "years set in the future" — with the bottom portion — "years set in the past". Each of these sections is in turn divided into two related subsections.
A link to a much larger version can be found by clicking here.
In the "future" section, the chart divides stories according to whether they're either "obsolete" or "still possible." A story is obsolete if we've already passed the timeline in which the story takes place. The classic example is Terminator 2, which predicted that Skynet would come online on August 29, 1997. As the chart shows, we're now 18 years ahead of that timeline, while the movie itself came out in 1990, seven years prior to the film's predicted date. Star Trek, by contrast, has yet to happen.
The "past" section is a bit different. The line represents the limit in which the publishing time is nearer to the time it takes place than to the present. Or as Munroe puts it, the dividing line indicates "stories written x years ago and set 2x years ago."
For example, as of today, we are further from the publication date of the film The Bridge on the River Kwai (released in 1957, for a difference of 58 years) than to the length of time the film is distanced from the events in the story (which occurred sometime around 1942 to 1943, for a distance of 14 years). Items in grey are "stories set in the past, but created long enough ago that they were published closer to their setting than to today." Consequently, "modern audiences may not recognize which parts were supposed to sound old."
Got it? If not, this page can help, and it also includes more information about the works listed in the chart.
[ xkcd ]