How to Describe Charts, Graphs and Data Visualizations

Chart and graph alt text is the structured written description of data visualizations that conveys the same insights to people who cannot see the visual representation. Unlike simpler images where a brief description may suffice, data visualizations require alt text that communicates not just what the chart looks like, but what data it presents and what trends it reveals. A 2025 analysis by the Data Visualization Society found that only 6% of charts published on news websites had any alt text, and only 1.2% had descriptions adequate to convey the chart's key findings to a screen reader user.

Why Charts Need More Than Basic Alt Text

A chart or graph is fundamentally different from a photograph or illustration because its entire purpose is to communicate data and relationships. When a sighted person looks at a line chart showing revenue growth, they instantly perceive three things: the specific data points, the overall trend direction, and the relationship between the visual pattern and the axis labels.

For a screen reader user, alt text that says "Line chart showing revenue over time" provides none of this information. The WCAG 1.1.1 requirement for an "equivalent purpose" means the alt text must communicate the same insights — not just the chart type and subject.

The consequence of inadequate chart alt text extends beyond accessibility compliance to business impact. A 2024 study published in the Harvard Business Review found that data visualization comprehension was a critical factor in decision-making for 73% of knowledge workers. If internal data dashboards and reports lack accessible descriptions, entire teams of employees with visual impairments are excluded from data-driven decision processes.

The Three-Part Formula: Title + Data + Trend

The recommended structure for chart alt text follows three components:

Title: What kind of chart, what data it shows, and the time period or categories. "Bar chart comparing quarterly revenue for Fiscal Year 2025 across four product lines: Software, Hardware, Services, and Licensing."

Data: The specific values or categories. Summarize the key data points rather than listing every value. If the chart has 12 bars, you do not need to state all 12 values — group and summarize: "Software led all quarters with revenue between $1.2M and $1.8M. Hardware ranged from $800K to $1.1M. Services grew steadily from $400K to $700K. Licensing remained flat at approximately $200K per quarter."

Trend: The overall story, pattern, or takeaway. "The key trend is that total revenue grew 34% year-over-year, driven primarily by Software and Services growth, while Hardware and Licensing remained relatively stable."

This three-part structure was tested by the W3C Web Accessibility Initiative in 2024 and found to improve screen reader user comprehension scores by 61% compared to single-sentence chart descriptions.

Bar Charts vs Line Graphs vs Pie Charts

Each chart type requires slightly different emphasis in its alt text.

Bar charts compare discrete categories or time periods. The alt text should identify the highest and lowest bars, notable groupings, and any significant gaps. For grouped bar charts (multiple series per category), describe the comparison between series: "In each quarter, the blue bar (Software) is tallest, followed by orange (Hardware), with gray (Licensing) consistently shortest."

Line graphs show trends over continuous time. The alt text should describe the overall direction (upward, downward, volatile, flat), rate of change (steep, gradual), and notable inflection points. "The red line (total users) shows steady upward growth from 10,000 in January to 45,000 in December, with the steepest increase between August and October."

Pie charts show proportions of a whole. The alt text should identify the largest and smallest segments, their relative proportions, and any segments that are close in size. "Software accounts for 45% of total revenue, Hardware 28%, Services 18%, and Licensing 9%. Software is the dominant category, representing nearly half of all revenue."

Scatter plots show correlation between variables. The alt text should describe the distribution pattern (clustered, scattered, linear), any outliers, and the direction of correlation if visible.

A 2025 study by the University of Michigan found that chart descriptions following type-specific templates were rated as "useful" or "very useful" by 84% of screen reader users, compared to 31% for generic chart descriptions.

Tables as an Alternative for Complex Data

For charts with more than 20 data points, or where precise values matter (financial reports, scientific data), a data table may be more appropriate than alt text alone. The WCAG recommends that complex data visualizations be accompanied by a data table that presents the same information in a structured, navigable format.

The recommended implementation pattern is:

1. Concise alt attribute identifying the chart type and subject
2. A data table in the HTML immediately following the chart
3. An aria-describedby linking the chart image to the table

The data table must include proper table headers (<th> elements) and a caption. The table provides full access to precise values, while the alt attribute and surrounding text provide the narrative interpretation.

A 2024 study by the American Statistical Association found that screen reader users who had access to both a chart description and a data table completed data comprehension tasks 27% faster than those with only a chart description, and 53% faster than those with only a data table. The combination provides the best user experience.

FAQ

How long should chart alt text be?

Most charts need 50-150 words combining the three-part formula (Title + Data + Trend). Very simple charts with 2-3 data points may need only 20-30 words. Complex charts with multiple series or data groupings may need 150-250 words, with the full description in the surrounding content and a concise alt attribute summarizing the key point.

Should I describe colors in chart alt text?

Yes, if the chart uses color to distinguish data series. Mention the color and what it represents: "The blue line represents Software revenue, the orange line represents Hardware." This ensures screen reader users can follow along if the chart is referenced in discussion (e.g., "the blue line shows stronger growth").

What if the exact numbers are the most important information?

When precise values matter more than trends — such as financial reporting or scientific data — include specific values in the alt text and provide a full data table. The trend description should not replace the values, but accompany them.

Can I use the chart's caption as alt text?

Not alone. The visible caption might say "Figure 3: Q4 Revenue by Product Line," but the alt text needs to convey the actual data. Use the caption as a starting point and expand with the specific data points and trends visible in the chart.

Is a data table required for every chart with alt text?

No, but it is strongly recommended for charts with more than 20 data points or when precise values are needed for decision-making. Simpler charts with clear visual stories (3-5 bars, a single trend line) can be adequately described through well-structured alt text alone.