Album de Statistique Graphique - Recensement de la Batellerie sur les Voies Navigables en 1887. I. Nombre de Bateaux Ordinaires et a Vapeur Recenses par Voie. (Number of ordinary boats and steam boats per inland waterway in 1887.) [12519.020]
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
Fixing the four tints in a key before the map is read makes a four-category comparison legible on a single line, because the reader is decoding a declared system rather than guessing at colours. Putting the rare category - steamers - in red against a blue mass exploits hue contrast to keep a small number visible next to a large one. The leader lines and ellipse tying each inset to its source location remove the usual cost of insets, which is losing the reader's place. The running heads make the plate self-identifying once it is unfolded away from the book.
Steal this when
Use this when a census or inventory has to be shown by location and by type at once, and the smallest category is the one that matters. Copy the tied-back inset - leader lines from the detail box to the exact spot it enlarges - any time you magnify a crowded region.