A Tally Chart is simultaneously a data collection method and a visual display. The tally mark numeral system — four vertical strokes crossed by a diagonal — groups counts in fives, making totals readable at a glance without arithmetic. Historically made with pen and paper, the form conveys something that no bar chart can: the process of counting. Each mark is a discrete event. The chart does not abstract the data into a continuous scale; it preserves the unit nature of what was counted. The animation here restores that accumulative process — marks appear as they would be drawn, one incident at a time.
The data is a count of discrete events (reported incidents) across a small number of categories with relatively modest counts (8–31 per category). A bar chart would be more precise, and a dot plot would be more elegant. But the tally form makes an argument that precision does not: these are individually countable events, not a measured quantity. Every mark is an incident that was filed by a real person. The form grounds the data in its origin — a manual counting process — which is appropriate when the act of counting is itself part of the story. When counts exceed ~60 per category, the form breaks down and a histogram is correct.
A histogram bins continuous values into intervals and shows their frequency as bars. A tally chart counts discrete events in categorical rows. They look similar when complete — the Data Visualisation Catalogue notes that "the final result is similar to a histogram" — but they are conceptually different. The histogram's x-axis is a continuous scale; the tally chart's rows are independent categories. You cannot use a tally chart for continuous data (temperature, time-to-resolution) without first binning it, at which point a histogram is the correct form. The tally chart's limitation is also its strength: it only works for countable, discrete data.
FT Visual Vocabulary: Distribution — Frequency (closest match; tally charts predate most formal charting frameworks). Abela quadrant: Comparison — comparing discrete frequency counts across categories. Tufte: tally charts achieve near-perfect data-ink ratio — every mark is one data point.
The one decision worth knowing: phantom group boxes (faint rectangles) are drawn before the marks animate in. This mirrors the pre-printed form structure of a physical tally sheet, where blank boxes exist before any data is recorded. The boxes also make the group-of-5 structure legible from the first frame — the viewer understands the encoding before a single mark appears.