Mouvement des marchandises en Belgique sur les chemins de fer en 1844 et sur les voies navigables en 1834 et en 1844
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
Three variables share one mark without collision: position gives the route, width gives the tonnage, colour gives the change between two years. Because gain and loss are tints stacked inside the same band, the reader compares two dates without comparing two maps, which removes the eye movement where most before-and-after graphics lose their audience. The note carries the arithmetic of one worked example, so the reader is trained to decode every other band on the sheet. The conclusion, that coal is more than half the water-borne tonnage, is only stated in the margin after the picture has already made it.
Steal this when
When you need to show change between two periods and a side-by-side pair would force the audience to hold one image in memory, encode the shared part, the gain and the loss as colour inside one shape.
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