Map of cholera deaths in the Broad Street / Golden Square outbreak, London - the original 1854 map
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
The sheet holds almost nothing back for decoration: everything except street outline, deaths, pumps and one boundary is stripped out, so the cluster has no competition for attention. Bars stacked on the frontage make frequency into physical length, and because they accumulate outward from the building line, the worst addresses read as mass rather than as a number to be looked up. Setting PUMP in the heaviest weight on the page makes the candidate cause the second thing the eye finds, immediately after the black mass. The irregular dashed boundary is the argument made visible: it draws the territory where Broad Street is the nearest pump, and the deaths sit inside it.
Steal this when
Copy this when you have incident-level data and a suspected cause, and the audience already believes something else. Show every incident at its true location, show all the candidate causes and not only yours, and strip the page down until the pattern is the only thing left on it.