Charts, maps and diagrams: extended descriptions

A chart cannot be squeezed into alt text. Give it a short alt naming what it is, and put the data in an extended description the reader can choose to open.

The structure

• One sentence naming what the image is and how it is organised. For a chart: what runs on each axis, and in what units.
• What it shows: the values and patterns actually visible. If a value is not legible, say "the exact values are not legible" rather than estimating.
• The takeaway: the single point a sighted reader takes from it, if there is a clear one.

How Reflow ships it

The description is written into the file as a details disclosure at the end of the chapter, linked from the image with aria-details. That insertion is structurally valid in EPUB 3 and survives the EPUBCheck gate, which is why we chose it over the older longdesc attribute.