How to Write Alt Text for Complex Charts and Data Visuals
Charts, graphs, dashboards, and infographics can communicate a lot of information all at once. That is exactly what makes writing alt text for them a little more complicated.
With a standard image, you may only need a short description. With a chart, someone may need to understand what is being measured, what changed, how categories compare, and what conclusion the visual is trying to communicate.
Trying to squeeze all of that into an alt text field usually does not work very well.
Instead, accessible data visuals typically need two things: a short text alternative that identifies the visual and communicates its main takeaway, plus a way to access the important details that do not fit comfortably in the alt text.
In this article, we’ll look at how to decide what belongs where, what to avoid, and how to write useful alt text for different types of charts and data graphics.
In This Article
Why Complex Charts Need a Different Alt Text Approach
What Good Alt Text for Charts Is Actually Supposed to Do
How to Decide What Belongs in the Alt Text
A Formula for Writing Alt Text for Complex Charts
What to Include in the Longer Description or Nearby Text
How to Design More Accessible Data Visualizations
Examples of Better Alt Text for Different Chart Types
What to Ask Before Sharing Your Data Visuals
Frequently Asked Questions About Alt Text for Charts & Data Visuals
Why Complex Charts Need a Different Alt Text Approach
Writing alt text for a photo is usually pretty straightforward. You identify what matters in the image and describe it briefly. A cat on a windowsill is easy. A quarterly revenue bar chart is not.
That is because a complex chart is doing more than showing an object. It is presenting information, comparisons, trends, and relationships.
W3C guidance treats graphs, charts, diagrams, and similar visuals as complex images because they can contain more information than reasonably fits into a short text alternative. In those situations, users need access to the essential information in another form as well.
This is where people often get stuck. They know the chart needs alt text, so they try to squeeze the entire visual into one enormous description.
The result tends to be either too vague to be useful or so detailed that it becomes difficult to follow.
A better approach is to give the alt text one clear job: identify the visual and communicate the main takeaway. The fuller explanation can then live in surrounding text, a caption, an accessible data table, or another longer description.
What Good Alt Text for Charts Is Actually Supposed to Do
For a complex chart, useful alt text should quickly help someone understand
What type of visual is this?
What is it about?
What is the main takeaway?
Your alt text shouldn’t sound like a spreadsheet being read out loud. Think of it more like a useful summary.
For example, this would not be helpful
“Bar chart with blue, green, and orange bars for Q1, Q2, Q3, and Q4 showing revenue in dollars.”
It tells the user that a chart exists, but that’s about it.
This would be more useful
“Bar chart of quarterly revenue showing steady growth across the year, with Q4 as the highest quarter.”
Now the user knows both what the chart represents and the main conclusion someone looking at it would probably notice.
If exact numbers or additional comparisons are important, those should also be available in accessible text somewhere nearby.
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A simple way to think about this is to separate the main message from the supporting detail.
The main message belongs in the alt text.
The supporting detail belongs in the surrounding content.
Imagine explaining the chart to someone over the phone. What would you tell them first before going into all the numbers? That first sentence is often pretty close to your alt text.
For example, imagine a line graph showing website traffic for six months.
The alt text might say
“Line graph showing website traffic rising each month from January to June, with the largest jump in May.”
Then the text below the chart might explain
“Traffic increased from 4,200 visits in January to 8,900 in June. Growth was gradual through April, then accelerated in May after the site redesign.”
Someone using a screen reader gets the main point immediately, while the more detailed information is still available when they want it.
This approach is also useful beyond accessibility. If you cannot explain what a chart is trying to communicate in one reasonably clear sentence, the chart itself may be trying to do too much.
A Formula for Writing Alt Text for Complex Charts
When you are not sure where to start, try this simple formula
Chart type] + [topic] + [main insight
For example
“Bar chart of quarterly revenue showing consistent growth, with Q4 nearly double Q1.”
“Pie chart of traffic sources showing organic search as the largest source by a wide margin.”
“Line graph of customer support tickets showing a sharp drop after the new help center launched.”
Notice what these descriptions are not doing. They are not listing every label, color, value, or decorative detail.
They are communicating the information someone needs in order to understand the purpose of the visual.
What to Include in the Longer Description or Nearby Text
Once your alt text communicates the main takeaway, the longer description can provide the information someone would get by spending more time examining the chart.
Depending on the visual, that may include
What is being measured
The time period
Categories or data series being compared
Important values
High and low points
Changes or patterns
Relationships between different sets of data
The conclusion the chart is intended to support
Often, the simplest solution is to put this information directly on the page where everyone can access it.
That might mean adding
A useful caption
A short paragraph below the chart
An accessible data table
A longer written explanation for particularly complex information
A link to a full report or page with more details
If exact values are important, a data table is often especially useful. Someone can review the numbers directly rather than trying to pull every value out of a long alt text description.
How to Design More Accessible Data Visualizations
Alt text and supporting descriptions are only part of making charts and data graphics accessible. The visual itself should also be as easy to understand as possible.
A well-designed chart can reduce the amount of explanation people need because the information is organized clearly from the start.
Here are a few things to keep in mind when creating data visuals.
Label Data Clearly
Whenever possible, label data directly instead of making people constantly look back and forth between the chart and a separate legend.
For example, labeling each line in a line graph with the name of the data series can be easier to follow than relying only on a color-coded legend.
Axis labels, units of measurement, dates, categories, and other important context should also be clearly identified.
Do Not Use Color as the Only Way to Show Differences
Color can be useful, but it should not be the only thing separating one category from another.
Combine color with another visual difference, such as:
Patterns or textures
Different line styles, such as solid, dashed, or dotted
Shapes or symbols at data points
Direct text labels
For example, two lines on a graph could use different colors and different line styles. Someone who has difficulty distinguishing the colors can still tell the lines apart.
Check Color Contrast
Text, labels, icons, and important graphical elements need enough contrast against their backgrounds and surrounding colors to remain visible.
For charts and graphs, this includes things like lines, data points, chart boundaries, and other graphical elements that someone needs in order to understand the information.
Do not assume two colors are distinguishable just because they look different to you. Similar shades can become especially difficult to distinguish for people with low vision or color vision deficiencies.
Keep the Visual Focused
More data does not automatically make a chart more useful.
Too many categories, colors, labels, gridlines, decorative elements, or data points can make the important information harder to find.
Ask yourself what someone actually needs to learn from the visual.
If several pieces of information are not helping tell that story, consider removing them or breaking the information into multiple simpler visuals.
Use Readable Text
Chart titles, labels, legends, and data values should be large enough to read comfortably.
Avoid squeezing large amounts of information into tiny text just to make everything fit into one graphic.
This is especially important when charts may be viewed on smaller screens or reused in presentations, social media graphics, reports, or other formats.
Give the Chart a Clear Purpose
A good chart should make it fairly easy to answer the question
What am I supposed to learn from this?
Use a clear title, logical organization, and visual emphasis to help people identify the important information. Then make sure that same takeaway is also available through the alt text and supporting content.
Accessibility works best when the visual design and the text alternative are supporting the same message.
Examples of Better Alt Text for Different Chart Types
Bar Chart Example
Not very helpful: “Bar chart with sales data.”
Better: “Bar chart of monthly sales showing a dip in February followed by steady growth through June.”
Supporting text: “Sales fell from $18,000 in January to $15,500 in February, then rose each month to reach $26,000 in June.”
Line Graph Example
Not very helpful: “Line graph with two lines.”
Better: “Line graph comparing desktop and mobile traffic, showing mobile traffic overtaking desktop traffic in April.”
Supporting text: “Desktop traffic stayed mostly flat through the first quarter, while mobile traffic climbed steadily and became the leading source in April.”
Pie Chart Example
Not very helpful: “Pie chart of marketing channels.”
Better: “Pie chart showing organic search as the largest marketing channel, accounting for about half of total traffic.”
Supporting text: “Organic search drove 49% of visits, followed by direct traffic at 24%, social at 15%, and email at 12%.”
What to Ask Before Sharing Your Data Visuals
Before publishing a chart or data graphic, ask
What is this chart actually trying to communicate?
What information would someone miss if they could not see it?
Can I explain the main takeaway in one sentence?
Are important numbers and details available as text?
Would the page still make sense if the image did not load?
Can we remove some data to make the visual more impactful?
Those questions can help you create both better alt text and better data visualizations.
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Frequently Asked Questions About Alt Text for Charts & Data Visuals
Do Complex Charts Need Alt Text?
Yes. Charts and graphs that communicate meaningful information need an appropriate text alternative. With complex visuals, a short alt text description is often paired with a fuller explanation of the important data elsewhere on the page.
Should Alt Text Include Every Number in a Chart?
Usually not. If every number is necessary to understand the content, provide those values in accessible surrounding text or a data table instead. Use the alt text to summarize the main information being communicated.
What Is a Good Format for Chart Alt Text?
A useful starting point is: Chart type + topic + main insight For example: “Line graph of monthly website traffic showing steady growth after the redesign.”
Where Should the Full Chart Description Go?
Depending on the amount of information, it might appear in: A caption Text immediately before or after the chart An accessible data table A longer description linked from or associated with the visual Choose the format that makes the information easiest for people to access and understand.
Is a Data Table Better Than a Long Description?
Sometimes. If exact values and comparisons are important, an accessible data table can be one of the clearest ways to provide the underlying information. A short written summary can then explain the overall trend or takeaway.
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