Album de statistique graphique de la Ville de Paris - sheet 1 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
Encoding two variables on the two sides of a rectangle so that the area is their product lets one mark carry count, unit value and total at the same time, which is why the sheet can answer three questions per branch without three charts. Pinning each mark to its real address turns the statistic into geography: the reader sees immediately that the eastern working districts lend small sums often while the western bureaux lend larger sums less often. Colour is doing categorical work, rose for pledges, yellow for renewals, cross-hatch for redemptions, so three related transaction types stack at the same location without a second panel. Printing the whole underlying table on the same sheet means the graphic never has to be trusted on faith, and the legend states the exact millimetre-to-unit conversion so any measurement can be redone with a ruler.
Steal this when
Use this when a metric is really the product of two others, volume times average value, and stakeholders keep arguing about which one moved. Draw it as an area, put the two drivers on the two sides, anchor each mark to the place or account it belongs to, and print the source table underneath.