When the chart is the only place the numbers live, one sentence of alt text is not enough
One sentence of alt text only says that the image is a chart. When the chart is the only source of the numbers, people who cannot see it need a long description, and where it goes depends on the page.
You have just finished a bar chart of revenue at three stores over the first three months of the quarter. You paste it into a chatbot and ask it to “write the alt text” (the replacement text for an image, which a screen reader reads aloud when the user cannot see the image). The draft that comes back: “Bar chart showing revenue of three stores by month.” That sentence isn’t wrong. But a person who cannot see the chart still doesn’t know which store is rising and which is falling.
For a product photo or an illustration, one short sentence is usually enough. A chart is the kind of image where the numbers live inside the picture, so a short sentence can only say “what kind of chart this is”. The rest has to go somewhere else, and where depends on the page you use the chart on.
The example below was built by the editors to illustrate how to decide. The figures are made up; they are not the output of a chatbot or of a real store.
The source table
Monthly revenue, in millions of dong:
| Month 1 | Month 2 | Month 3 | |
|---|---|---|---|
| Store A | 120 | 110 | 95 |
| Store B | 80 | 80 | 81 |
| Store C | 50 | 70 | 90 |
A bar chart drawn from this table will be used in two places.
Place one: a report that already states the result
In the quarterly report, right below the chart there is a paragraph: “Store A fell from 120 to 95 million, store B stayed around 80, store C rose from 50 to 90.” A reader using a screen reader hears this paragraph like everyone else.
Here the chart illustrates what the paragraph already says. A short alt sentence is enough, as long as it points to the paragraph: “Bar chart of revenue at three stores, month 1 to month 3; the figures are given in the paragraph below.” Copying the whole table into the alt text only makes the listener hear the same information twice.
Place two: a slide with only a title and a chart
On a slide sent to the whole company, the title is “Revenue by store” and below it there is nothing but the chart. Take the chart away and the reader has nothing left. This is a complex image in the W3C WAI classification, a group that includes charts, diagrams and maps. W3C advises a text alternative in two parts: a short part saying what kind of image it is, and a long part that presents the essential information in words, including the trend if the trend is what the chart is meant to show.
For this slide you need both. Short alt: “Bar chart of revenue at three stores, month 1 to month 3; detailed description below the chart.” Put the long description right on the page, under the chart, so everyone can read it, rather than cramming it into the alt field. W3C recommends making the long description visible to everyone and linking it to the image with a line of text that says where it is.
Using AI for the long description
Don’t give the chart image to the AI and let it read the numbers off the bars. Give it the source table above, which you already have, and state the description’s role: “This is a long description for a bar chart placed on a slide. State the value for each store and the main trend. Use only the numbers in the table; do not explain causes.”
Checking what the AI returns
Read the draft and check every number against the table. Then cover the chart and ask: can someone who has only the text say that A fell, B stayed flat and C rose? If an idea is missing, add it. If the draft adds a sentence like “because store C ran a promotion”, delete it, because the table says nothing of the kind.
If your slide tool has only one alt field and doesn’t let you add text under the image, put the table right next to the chart. A small table is also a long description, and screen reader users still get the figures.
What this article does not replace
This is a quick check for content makers, not a full accessibility audit. For a chart with many data series or a complex structure, see W3C WAI’s own guidance on complex images and the standards your product has to meet.
Sources
- W3C WAI — Complex Images — explains the two-part text alternative for charts, diagrams and maps, and the ways to place the long description on the page.
- W3C WAI — An alt Decision Tree — guides the choice of text alternative by whether the image contains text, has a function, carries information or is only decorative.
- W3C WAI — Images Tutorial — explains that a text alternative depends on the image’s purpose, context and content.