Long descriptions of charts should capture data trends, not only visual appearance

What it is

“The blue series sits above the orange series, with a legend in the top right” describes how a picture looks. Someone who cannot see the picture needs whether the series rose or fell, which is higher, where it inflects, whether there is an outlier. A long description of a chart should hand over trends and comparisons, not recap colour, stroke and decoration.

A short alternative name is still needed, so a list can identify “what kind of chart this is.” The short name is not responsible for finishing the trend; stuffing the trend into the short name produces an appearance paragraph nobody can listen through.

Why it happens

A chart translates position, length and angle into quantitative relations. Sighted reading takes the trend in a few sweeps: slope first, then crossings, then extremes. Without those sweeps, the same trip has to be rebuilt from language. If the language stops at appearance (“three lines, the blue one thickest”), what is rebuilt is the drawing program, not the dataset.

Trend writing is a selection: not every point read aloud, but the structure a reader would take first — overall direction, main comparison, exceptions. A point table is a different channel; a long description that becomes a spoken point table overloads working memory and loses the trend.

Appearance vocabulary also misleads. Colour vision, greyscale print and dark themes rewrite a sentence like “blue above orange”; a trend (“series A stays above series B throughout”) still holds under those conditions.

Studying it

Write two long descriptions of the same chart: appearance only, versus trend and comparison. Ask screen-reader users (or participants with the figure masked) pre-written data questions: which is higher, which year is the turn, is there an outlier.

Independent variables: description type (appearance / trend), presence of a separate point list. Dependent variables: accuracy on the key questions, time to finish, whether participants ask for a repeat.

Questions must be written before the figure is seen, so authors cannot set items the description happens to support. For complex figures, also test whether listeners can restate the overall direction — a better proxy for whether the trend arrived than per-point accuracy.

Where it stops holding

For diagrams, flowcharts and floor plans the “trend” is not a time series; the long description should be structural (who connects to whom, which step follows a branch), not forced into rise-and-fall. Photographs and illustrations have no dataset; appearance or scene is the right object, and this chart criterion does not apply. Exploratory sketches whose authors have not yet committed to a conclusion become over-interpretation if given a trend sentence; mark “direction not yet judged” rather than invent a slope. A sparkline is a deliberately tiny trend mark; one nearby sentence is usually enough, and it does not need an essay.

Applying it

• Write first the three things a reader would take from the figure: overall direction, main comparison, exceptions that need their own clause. Colour and stroke are secondary labels, used only to tell series apart.
• Do not lead with “a thick blue line from top-left to bottom-right.” Lead with “from 2020 to 2024 series A stays above series B, with a brief crossing in 2022.”
• Keep the short alternative name for type and object (“quarterly revenue line chart”); leave the trend to the long description.
• How to check: without looking at the figure, listen to or read the long description and answer three pre-written data questions. Beautiful appearance prose that cannot answer the trend items is a fail.

Related

• Same group: J2.12.2 A long description can sit in nearby visible text or on a linked page · J2.12.3 Interactive charts need an equivalent data table as an alternative presentation · J2.12.4 Writing a long description requires understanding the data behind the graphic, not a post-hoc caption
• Nearby: J2.04 Alternative Text · J2.03 Color is Not the Only Cue · J4.03 Audio Description
• Search terms: long description · chart trends · non-visual data summary