Chart description

A chart carries two separate things a text alternative has to deliver, and most alternatives deliver neither. The first is the conclusion the chart was drawn to support. The second is the numbers that support it. The shape that works is a one line summary naming the takeaway (“support tickets fell by a third after the March release”) followed by real access to the values, normally a data table sitting beside the figure or behind a disclosure, marked up as rows and columns a screen reader can walk. The W3C’s complex images tutorial calls this pairing a short description plus a long description, and it is the only arrangement that survives a chart with more than one series.

Alt text is the shorter word for the shorter job, and the two are worth keeping apart. Alt text is the string an image carries, and for a chart it can hold the summary half at best, which is why a chart description usually cannot live in an attribute at all. Going the other way: writing alt="bar chart of quarterly revenue" describes the picture, and describing the picture is exactly what a chart alternative must not do, because a reader who cannot see it did not ask for the pixels to be recited. Say what the chart shows, then hand over the numbers. Audio description sounds like a relative and is not one: that narrates a moving picture in the gaps of its own soundtrack, while a chart description is static text that the reader controls the pace of.

None of the chart’s own furniture carries across the boundary by itself. An axis is a ruled edge with tick labels a reader can see, a chart legend is a colour key nobody can hear, and direct labeling puts names next to lines that are also unavailable. A chart annotation is the exception worth exploiting: the note already pinned onto the data, saying what to see, is usually the best first draft of the summary line, because somebody has already done the work of deciding what the chart is for.

The practical test is to read the alternative aloud with the chart covered. If the listener cannot repeat the takeaway, the summary is missing. If they cannot answer a question about a specific point, the values are missing. A description that passes both is worth building into the page for everyone: sighted readers use the table too, and a sparkline or a small multiple with a table under it is a better artefact than either half alone.