Charles Joseph Minard's famous visual telling of Napoleon's 1812 invasion of Russia is one of the first great examples of an infographic, if a little intimidating at a glance. And this explanation by the folks at Numberphile really helps bring it home.
What's really remarkable about this graph is the crazy number of variables it manages to keep track of while staying lucid. A total of six, including the size of Napoleon's army, the direction it was headed, the temperature on various days, and even off-shoot sorties from the whole doomed campaign.
Of course until you have some explanation it just looks like a weird line, and it doesn't help that the description is written in French. Fortunately the Numberphile walkthrough is more than detailed enough to help you learn a little about history; the history of infographics and the history of why not to invade Russia in the winter. [Numberphile]