Pair an identifiable chart with a meaningful summary and usable underlying values; keep those alternatives synchronized with the plotted data and controls.
Explain what a chart helps the reader understand, then make its important values and relationships available without requiring sight of the graphic. A label such as “line chart” identifies a format but does not convey the finding. A spreadsheet of values alone may also leave the reader to reconstruct the intended argument.
Consider a fictional library’s chart of monthly repair-workshop bookings. It shows 20 in January, 35 in February and 30 in March. The editorial point is that bookings rose in February and remained above January in March. These invented figures illustrate the workflow; they are not measured website results.
Start with the question the chart answers
Write the reader’s question before writing alternative text. Is the chart comparing categories, showing a trend, locating a threshold or explaining a distribution? Identify the specific relationship the article relies on. If the graphic does not help answer a useful question, consider simplifying or removing it.
W3C’s Non-text Content explanation calls for an alternative serving the equivalent purpose. Its chart example distinguishes a short label from a longer account of values, trends and implications, with underlying data where practical. The purpose determines what information needs to survive.
For the library, a useful summary is “Bookings peaked at 35 in February, then eased to 30 in March; March remained 10 above January.” That tells the relevant story without pretending that three months establish a lasting growth pattern.
Use layers instead of one enormous label
W3C’s complex-image guidance describes short identification and a longer representation of essential information. For charts, that can include scales, values, relationships and trends. Make the extended explanation discoverable near the graphic rather than hiding the entire analysis in one lengthy accessible name.
The short identification can name the topic and direct readers to the nearby explanation. The visible summary can state the main relationship. A data section can then provide exact values, units and scope. Different readers can stop at the layer that answers their question.
The image-alt-text guide covers context and purpose more broadly. Here the goal is an analytical equivalent, including information a reader would otherwise infer from positions, axes or plotted relationships.
Preserve the numbers behind the shape
Record the unit, period, denominator where relevant, source and treatment of missing values. A zero is not the same as missing data. A percentage without the population it describes can suggest a comparison the original data does not support.
For the library, state that the numbers are completed bookings by calendar month, not unique attendees or available seats. If February has a shorter reporting period or March is incomplete, that qualification belongs beside both the chart and the values.
| Fictional month | Completed bookings | Relationship to January |
|---|---|---|
| January | 20 | Baseline in this example |
| February | 35 | 15 more |
| March | 30 | 10 more |
This small table makes the exact comparisons available. It does not establish the reason for the February peak. Keep observation and explanation distinct: a promotion might be a hypothesis, but the booking counts alone do not prove it caused the change.
Explain axes and visual choices that affect interpretation
If a chart uses a logarithmic scale, a truncated axis, a moving average or a cumulative total, explain the choice in language relevant to the reader. Do not assume a data export automatically communicates what the plotted shape means.
For a bar comparison, a shortened axis can make a modest difference look dramatic. The equivalent explanation should preserve the actual magnitude, not repeat a visual impression as fact. If the chart shows a range or uncertainty band, identify what it represents and avoid converting it into a precise guarantee.
For the three-month example, ordinary counts are enough. Adding a trend line through three points would create an extra claim to explain. Keep the graphic proportional to the evidence and retain the limits in its summary.
Make the underlying table usable
W3C’s table guidance distinguishes a caption identifying the table from a summary explaining its organisation. Use meaningful headers and appropriate structure rather than a visual grid made from unrelated text boxes. Avoid repeating the same sentence as both name and description.
The mobile table guide covers readable layouts. Ensure the chart alternative remains usable on a small screen: a wide table hidden behind clipping is not a dependable route to the values.
For a large dataset, provide a meaningful on-page summary and a clearly described way to inspect the relevant values. A downloadable file can supplement access, but it should not be the only place where the article’s basic conclusion becomes understandable.
Keep interactive alternatives synchronized
If readers filter by location or period, the summary and values need to change with the graphic. Otherwise a screen reader may receive the original quarter’s explanation while the visible chart shows another. Name the active selection and the resulting scope.
Do not make mouse hover the only route to a precise value. Review keyboard and touch access, the exposed control names and a meaningful route through the data. A text alternative for the static canvas does not create interaction semantics for the controls around it.
The sortable-table guide covers ordering real values and keeping rows intact. If sorting is available in the alternative table, ensure it changes a useful view without breaking the relationship to the current chart selection.
Check the smallest important comparison explicitly. In the fictional data, March is five below February but ten above January. A summary saying only that bookings fell loses the baseline relationship the article needs. Have a reviewer reproduce those simple differences from the table before approving the wording.
Test the article’s answer without the graphic
- Hide or ignore the chart and read its surrounding explanation.
- Answer the original analytical question using the summary and values.
- Check units, time range, missing data and uncertainty against the source.
- Follow the route to exact values with a keyboard and relevant assistive technology.
- Change filters and verify every representation uses the same selection.
- Repeat at a narrow width and with enlarged text.
Accept the result when readers can obtain the same important finding and check its basis. Your next step is to write the chart’s question, a concise answer and the smallest useful set of supporting values before deciding how to draw it.
Sources and verification
Editorial review 2026-10-17. Source check dates are recorded separately. Notes describe what each source supports and its limits.
View sources
- W3C: Complex ImagesChecked Short identification plus detailed equivalent information including chart values, scales, relationships and trends.
Limits: Does not prescribe one charting library or imply a raw data dump always communicates the same meaning. - W3C: Non-text ContentChecked Text alternatives serve equivalent purpose; data-chart example includes high-level meaning and practical underlying data.
Limits: One text alternative does not cover every interactive control or whole-site accessibility. - W3C: Tables Caption and SummaryChecked Captions identify a table; summaries explain organisation without unnecessary duplication.
Limits: Table structure guidance does not validate the accuracy of source data or a chart's inference.