Carte figurative des mouvements et provenances des cereales importees en France en 1867
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
Two inks split the dataset along the axis the reader actually cares about - mode of transport - so a single glance separates sea from land without any legend reading. The bands narrow as they pass each unloading port, which encodes depletion in the geometry itself rather than in a second chart. The price step-chart is placed as a companion, not a decoration: the map explains the volume and the inset explains the price that pulled it, and together they make a causal argument without a sentence of narrative.
Steal this when
Use two-colour flow when your audience keeps conflating two channels that behave differently - organic versus paid, direct versus partner, imported versus domestic. Pair it with a small companion chart of the price or driver when the point is not just how much moved but what moved it.