Most data visualizations are simple and utilitarian. This one is complicated and utilitarian:
The chart was the work of Charles Joseph Minard, a French civil engineer who spent his retirement years making innovative visualizations. The chart in question, “Carte figurative des pertes successives en hommes de l’Armée Française dans la campagne de Russie 1812–1813,” shows what its title describes: “successive manpower losses in the French Army’s campaign in Russia, 1812-1813.”
A broad strip, representing Napoleon’s army, stretches across the map before doubling back. You realize it represents the progress of his army. Then you understand why it gets thinner as it meanders across the map: French soldiers dying, day after day. You realize that the plot on the bottom shows the recorded low temperature on selected days.
The chart does what all great visualizations do: highlight an important pattern in a morass of data. It shows plainly why Napoleon lost the campaign. From the start, disease, starvation, and Russian resistance ate away at his army. The Russian army refused to oblige his plan to destroy it. By the time his diminshed forces marched unopposed into Moscow, their defeat was sealed. Most of those who remained succumbed to cold or Cossacks on the long retreat.
The ever-narrowing strip tells this brutal story more vividly than pages of tables or narrative prose could. (It also neatly dispels the misconception that the winter cost Napoleon the campaign. It just turned a fiasco into a catastrophe). Few modern analysts (me included) could come up with such an original and striking way of displaying the data. Minard’s work still stands as the best data visualization of all time.