Alt text should describe what the image conveys and why it matters, not what it depicts literally. For a data chart, state the finding and the key numbers. Decorative images should carry an empty alt attribute so screen readers skip them.
'Line chart showing AI-sourced sessions rising from 120 to 890 per month' beats 'chart'.
Alt text is also an AI extraction surface — a well-written alt hands the finding over in citable form.
Never stuff keywords; it degrades the experience for the people alt text exists to serve.