Maps should show a wide range of statistical data over geography and time. Statistics are sometimes too abstract to be useful much less educational. Visualizing it on a geographic map helps considerably.
Epidemiology and crime statistics are simplified by mapping out the data so spatial patterns can be analyzed. Political maps, likewise, should condense as much information as possble.
Here are two examples from Edward Tufte, a major advocate for statistical maps. One is John Snow’s Epidemiology map from 19th century London, and the other is Charles Minard’s Map of Napoleon’s failed invasion of Russia.
John Snow’s London Cholera Map
John Snow created one of the first epidemiology maps. Every dot represents the location of a death and the crosses represented water wells. This is how he isolated the Broadstreet well as the culprit in spreading the disease. By closing that well he contained the outbreak.
Charles Joseph Minard’s Map of Napoleon’s Advance and Retreat from Moscow
Tufte calls this one of the greatest statistical maps ever created. Minard showed how Napoleon’s Grand Army of half a million men left Poland and advanced to capture Moscow. The French Army encountered plague, desertions, combat, starvation and the brutal Russian Winter. The French Army dwindled to almost nothing. Minard’s map brilliantly condenses all this statistical information.