A Series of Statistical Charts Illustrating the Condition of the Descendants of Former African Slaves - Line graph shows comparison of African American to white populations between 1790 and 1890 [LCCN2013650372]
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 annotations are the technique worth stealing: the causal events are lettered vertically inside the data field at the exact year they act, so the chart makes a causal claim without a word of body copy and without leaving the graphic. Painting the series as thick ribbons rather than hairlines keeps both readable at exhibition distance and makes the widening gap a physical area rather than a divergence of two thin lines. Dropping the decade-by-decade percentages directly onto the bands gives the detailed reader the rate while the casual reader still gets the shape. The legend is bilingual inside the drawing, so the hand-lettered layer and the printed layer each carry both languages.
Steal this when
When a trend line only means something once the reader knows what happened to it - a pricing change, a regulation, a launch. Letter the events into the plot at their date rather than footnoting them below, and the chart argues cause instead of reporting shape.