How to Write Alt Text for Charts and Graphs

Charts and graphs present a special challenge for alt text. A chart's value lies in its data and trends, not just its appearance. This guide walks you through a proven method for writing alt text that conveys the essential information.

Why Charts Need Special Attention

When someone can't see a chart—whether due to low vision, using a screen reader, or a broken image—they should still understand:

• What type of chart it is (bar, pie, line, scatter, etc.)
• What the chart measures (title, subject)
• The key trend or insight
• Where to find detailed numbers (if needed)

Saying "A chart showing data" is not enough. Your alt text should be a summary a sighted reader would understand at a glance.

The Four-Part Method

Part 1: Chart Type

Start with what kind of chart it is. Examples:

• "Bar chart showing..."
• "Line graph tracking..."
• "Pie chart of..."
• "Scatter plot comparing..."
• "Stacked column chart depicting..."

This sets expectations for how to interpret the data. A line chart shows trends over time; a pie chart shows parts of a whole.

Part 2: Subject & Title

What does the chart measure? Include the chart's title or main topic:

• "Bar chart of Q1–Q4 revenue"
• "Line graph tracking website traffic over 12 months"
• "Pie chart of market share by competitor"

Part 3: Key Trend or Numbers

This is the heart of your alt text. What's the main insight? Include specific data points:

• For trend charts: "Revenue increased from $50k to $92k across four quarters"
• For pie charts: "Market share: Company A 45%, Company B 30%, Company C 25%"
• For comparison charts: "Product X (blue bar) sold twice as many units as Product Y (red bar)"

Include enough detail to make the chart useful, but keep it scannable. Aim for 100–150 characters when possible.

Part 4: Long Description Location (if needed)

For complex charts with many data points, your alt text can point readers to a longer explanation:

• "Chart of monthly sales by region; see table below for full data"
• "Network diagram showing software dependencies; detailed chart follows"

The long description might be a data table, a paragraph in the page text, or a linked spreadsheet.

Examples by Chart Type

Bar Chart

What you see: Vertical bars showing website traffic for four quarters.

Good alt text: "Bar chart of quarterly website traffic: Q1 50k visits, Q2 65k, Q3 78k, Q4 92k, showing steady quarter-over-quarter growth"

Why it works: Names the chart type, subject, specific values, and the insight (steady growth).

Pie Chart

What you see: A pie chart showing browser market share.

Good alt text: "Pie chart of desktop browser market share: Chrome 65%, Safari 20%, Firefox 10%, Edge 5%"

Why it works: Lists the categories and percentages in a clear order, making the proportions understandable.

Line Graph

What you see: A line graph tracking temperature over a year.

Good alt text: "Line graph of average monthly temperature: low of 32°F in January, peak of 86°F in July, returning to 35°F in December"

Why it works: Shows the range (low and high) and key months, giving a sense of the seasonal pattern.

Stacked or Multi-Series Chart

What you see: A stacked bar chart showing product sales by region.

Good alt text: "Stacked bar chart of product sales by region: North region $50k (40% Product A, 60% Product B), South region $30k (70% Product A, 30% Product B)"

Why it works: Breaks down both the main categories (regions) and sub-categories (products).

Scatter Plot

What you see: Points showing the relationship between age and salary.

Good alt text: "Scatter plot comparing age (x-axis, 20–60 years) and salary (y-axis, $30k–$150k); shows a positive correlation with tighter clustering above age 40"

Why it works: Explains both axes and the relationship shown.

Common Mistakes to Avoid

Mistake 1: Vague Alt Text

Bad: "Chart"

Better: "Bar chart showing Q1–Q4 revenue with growth trend"

Mistake 2: Describing Layout Instead of Data

Bad: "A bar chart with red bars on the left and blue bars on the right"

Better: "Bar chart comparing 2025 revenue (red, $92k) to 2026 revenue (blue, $115k), showing a 25% increase"

Mistake 3: Forgetting to Mention Trends

Bad: "Line graph with multiple lines"

Better: "Line graph comparing website traffic (blue line, steady growth from 50k to 150k) and email signups (orange line, plateauing at 5k)"

Mistake 4: Including Too Many Details

Bad: "Pie chart with six segments: A 25%, B 20%, C 18%, D 15%, E 12%, F 10%, showing precise decimal-point breakdowns for each category and axis labels..."

Better: "Pie chart of market share: Company A leads with 25%, followed by B (20%) and C (18%); three smaller competitors share the remainder; see attached spreadsheet for full data"

For many data points, a table or linked spreadsheet works better than cramming everything into alt text.

When to Use a Long Description

If a chart is so complex that you need more than 150 characters to convey it:

• Include a data table below the chart: Provide the raw numbers for readers who want detail.
• Add a caption: "Chart 1: Q1–Q4 revenue by product line. See Table 1 below for full breakdown."
• Provide a downloadable file: "Chart comparing 50 regional sales figures; CSV export available."

Tools & Best Practices

Step-by-Step Workflow

1. Identify the chart type: Bar, line, pie, scatter, etc.
2. Read the title: What is being measured?
3. Note the axes: What do X and Y represent? What are the units?
4. Identify key insights: What would a sighted reader glance at and notice first?
5. Write a one-sentence summary: Chart type + subject + key data + trend.
6. Count characters: Aim for 100–150; 125 is a good target.
7. Test with a screen reader: Does the meaning come through? Can the reader follow the insight?

Testing Your Chart Alt Text

Read your alt text aloud or use a screen reader (NVDA on Windows, VoiceOver on Mac). Ask yourself:

• Would I understand this insight if I couldn't see the chart?
• Are the most important numbers included?
• Is the language clear and concise?
• Do I know where to find detailed data if I need it?

Quick Reference

Formula: [Chart type] of [subject]: [key insight with data points], [trend or comparison].

Target length: 100–150 characters (roughly one sentence).

Always include: Chart type, subject, at least one key data point, and the main trend or relationship.

Consider adding a table/link if: The chart contains 5+ data series, precise decimal values, or context readers will need to verify.