Carte de la circulation des voyageurs par voitures publiques sur les routes de la contree ou sera place le chemin de fer de Dijon a Mulhouse
The pattern
This still is filed under Data as proof.
Define
A chart, map, or count that is the argument, not an illustration next to the argument.
Mechanism
Sighted evidence. The reader is asked to believe their eyes: a bar, a share, a spread on a map.
Verdict
The claim is comparative or quantitative and the figure can be read without a caption doing the sell.
Why it works
Position and quantity are encoded in the same mark: the band sits on the road, so where and how much are answered in one pass with no axis to read and no legend to look up. Stating the conversion rate in the cartouche turns the picture into an instrument, because a reader can lay a rule on the paper and recover the number. Everything that is not data is suppressed in weight, so rivers and town names cannot compete with the bands. The commercial argument is never written out; the thick band running Dijon to Besançon to Mulhouse is the argument.
Steal this when
When you want funding for something new and the demand already exists in an adjacent channel, plot the existing volume onto the map of the thing you want built and let width carry the case. Right whenever the audience distrusts a forecast but will accept observed traffic.