Album de statistique graphique de la Ville de Paris - sheet 3 of 12 (BnF btv1b52505495j)
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 double-encodes two variables that would normally need two charts: hue carries provenance while tint value carries the time period, so six series across twelve months fit in one field of view. The tip of every column is tinted to show the surplus itself, which converts a subtraction the reader would otherwise have to perform into a shape they simply see. Repeating the numeric axis on both flanks of each chart removes the long horizontal eye travel that makes tall bar charts misread. The legend is written as a reading instruction rather than a list of labels, so the encoding is taught before it is used.
Steal this when
Copy this when a recurring report has to set the current period against a rolling baseline across several categories at once, for an audience you cannot train first. The tinted cap on each bar is the specific move: render the delta as part of the bar instead of as a second chart.