Text Alternative for Charts

A text alternative is the written equivalent of a non-text element. WCAG success criterion 1.1.1 requires that “all non-text content that is presented to the user has a text alternative that serves the equivalent purpose” — and for a chart, the purpose is to convey a finding, not to announce that a picture exists.

This is where most chart alt text fails. “Bar chart of quarterly sales” identifies the image and conveys nothing a sighted reader would take from it. The equivalent purpose is the comparison: which quarter leads, by how much, and whether the trend holds.

W3C’s guidance treats charts as complex images needing a two-part alternative. The first part is “the short description to identify the image and, where appropriate, indicate the location of the long description”. The second is “a textual representation of the essential information conveyed by the image”, which for a chart includes “scales, values, relationships and trends that are represented visually”.

Two practical consequences follow. Write the trend, not only the structure — naming the axes without saying what happens along them leaves the reader with a labeled empty frame. And put the long description in the visible text rather than only in an attribute: W3C recommends making it available to everyone, and a written summary beside a chart reaches readers who never encounter alt text at all.

Publishing the underlying table alongside the chart serves the same end from the other direction. It gives a screen-reader user the values directly, and it lets any reader check the figure rather than trusting the rendering.

References:

• WCAG 1.1.1 Non-text Content
• W3C WAI, Complex Images