A chart with no alt text is a complete blank to a screen-reader user, not just a lesser view

What it is

Blind readers access web content through screen readers, which cannot interpret pixels—a chart without alternative text (alt text) is a complete blank to them. The key to alt text is not "describing what this chart is" ("a 2024 sales bar chart" serves no reader) but conveying the graphic's equivalent information: the key conclusion and key values the chart exists to communicate. "Equivalent" is the core accessibility concept—the text summary need not reproduce every detail of the graphic, but it must carry the same message-level content.

Why it happens

Alt-text effectiveness works through the alignment of information levels: a graphic's message can be layered—conclusion level ("sales grew every quarter"), pattern level ("a jump appeared in Q3"), and value level ("Q4 reached 1.2M"). An alt text that only describes chart type and axes stays at "physical description of the graphic," entirely misaligned with the conclusion level, leaving screen-reader users with near-zero information. Effective alt text writes the conclusion level and key values directly ("Sales grew each quarter in 2024; Q4 hit 1.2M, up 18% year over year"), translating visually encoded content into text-encoded content. The practical writing rule: lead with the title's conclusion, add one key figure, keep it to a sentence or two. More complex graphics (multi-series, exploratory) rely on the same group's data table for the complete equivalent, with the alt text serving only as a wayfinder ("quarterly detail in the table below"). Alt text also has secondary audiences: placeholder display when graphics fail to load, search-engine indexing, and any quick-scan context—good alt text improves the graphic's usability in all of them.

Where it stops holding

Alt text has length and duty boundaries: listening to overly long text (beyond a sentence or two) is a poor screen-reader experience, so long content belongs in long descriptions or data tables rather than crammed into the alt attribute. Purely decorative graphics (dividers, background art) should use empty alt (marked decorative, skipped from reading), not "decorative graphic"—reading decoration as content is itself noise. Alt text cannot carry interactivity (hover tooltips, drill-down); interactive information's alternatives depend on the same group's keyboard traversal. Finally, alt-text quality cannot be fully verified from code (automation checks "exists," never "is equivalent"), so equivalence requires a human review process.

Applying it

• Write conclusion-style alt text for every informative chart: one conclusion sentence plus one key figure, within two sentences.
• Complex charts' alt text acts as a wayfinder, pointing to the accompanying data table or long description.
• Use empty alt for all decorative graphics so screen readers skip them.
• Verification: turn off the screen, read only the alt text, and answer "what is this chart's conclusion?"; failure to answer means the alt text is not equivalent.

Related

• Same group: U11.01.2 The data table is the most reliable alternative form · U11.01.3 Interactive charts must support keyboard traversal
• Nearby: U9.02.2 Conclusion-style titles guide correct interpretation · U11.01.2 The data table is the most reliable alternative form
• Search terms: alt text · screen reader · accessible chart