Content and evidence
Make the evidence in an image available as text
Write useful alt text and visible chart descriptions, with a worked evidence table and a check for decorative and functional images.
TL;DR
- Use alt text to communicate the image's purpose in its page context.
- A complex chart needs its key information in readable text or a table as well as a short description.
- Keep the subject, metric, period and important limit close to the evidence.
- Do not assume every AI system is blind to images, or that adding alt text guarantees a citation.
If a chart is the only proof for a claim, a reader who cannot see or read it misses that proof. A short text description names the image. A visible note or data table gives the reader the facts.
That is a good reason to improve image content. It does not depend on guessing which model can read pixels. Tools differ. Even a tool that reads images may struggle with small labels, units or missing context.
Give alt text the right job
Google's image documentation describes using alt text alongside computer vision and page content. Alt text is useful context, not the whole image-understanding process.
For a complex image, W3C's guidance calls for a short description plus a longer text alternative containing the essential information. A chart with several series, a sample definition and a limitation usually cannot communicate all of that well in one alt attribute.
Separate those tasks. The short description identifies the chart and its main point. A caption locates the period and source. A nearby table or explanation carries the detail that someone needs to check the conclusion.
Worked example: a conversion chart with missing context
This example is fictional and uses invented data only to demonstrate the calculation. A service-page chart has the alt text “results graph” and a headline saying “Our page redesign improved conversion by 50%.” The underlying worksheet shows 20 accepted inquiries from 1,000 eligible visits in one period and 30 from 1,000 in the next.
Ask for the worksheet, the counting rules and the change log. They show what was counted, but do not separate the page edit from other changes. The headline claims a cause that the data cannot prove.
| Measure | Before period | After period |
|---|---|---|
| Eligible visits | 1,000 | 1,000 |
| Accepted inquiries | 20 | 30 |
| Accepted inquiries ÷ eligible visits | 2% | 3% |
| Change in rate | Baseline | +1 percentage point; +50% relative |
A better short description is: “Accepted inquiry rate increased from 2% to 3% across the two observed periods; data and scope follow.” The visible explanation should define an accepted inquiry and identify the periods, source worksheet and excluded traffic. It should also say that the comparison does not establish what caused the change.
The table now shows what each rate was based on. Readers can check the math and compare a point change with a relative change. The proof is easier to use, even if no search system ever cites the page.
The analyst checks the calculation. The editor checks that the headline, alt text and caption all describe an observed change. For the final check, hide the chart. Can a reader still find the result and its limit in the page text?
Treat different image roles differently
An informative photograph may need a brief description of the relevant product or condition. A linked icon needs a name that communicates its action or destination. A decorative flourish generally needs an empty alt attribute, not a keyword-filled sentence. These distinctions are covered by W3C's alt decision tree.
Context changes the answer. A photo of a valve on a product page may identify the variant. The same photo in an installation guide may need to explain the orientation that matters to the step. Reusing one generic alt string everywhere can miss both purposes.
Avoid putting unverified conclusions in alt text. “Industry-leading performance chart” adds a claim that the graphic may not support. Use the same evidence discipline as the visible body: describe what was measured, within its actual scope.
Keep evidence readable when assets change
Keep the source data and image text with the page record. When the chart changes, update its table, caption, conclusion and alt text too. Otherwise, the page can tell two different stories about the same result.
This work fits the Evidence dimension of Semanta's service-page audit framework. It makes the claim inspectable by a buyer, rather than relying on the visual confidence of a polished graphic.
Common questions
Can I put the entire data table in the alt attribute?
A visible table is usually easier to navigate and compare. Keep the short description useful, then provide the detailed equivalent in the body.
Does a screenshot of a table count as readable evidence?
It may help sighted readers, but it should not be the only access to key values. Publish the essential data as text and preserve its source.
If your strongest claims live only in images or slide exports, bring one example and its original data to discuss a Semanta evidence-content review.