How to Write Alt Text for Charts, Graphs, and Diagrams

Alt text allows people who cannot see images to access the information they convey. This becomes especially important for complex images like charts, graphs, and diagrams, because these visuals often communicate key insights, such as data trends, comparisons, or processes.

However, writing alt text for these visuals can be challenging. Unlike simple images that usually need only a brief description, charts and diagrams often contain detailed information, which can be difficult to describe clearly in text.

This guide explains how to approach it, covering practical ways to write alt text for charts, graphs, and diagrams, along with tools that can help generate descriptions for these complex images.

How to write alt text for complex images

To make complex images such as charts, graphs, and diagrams accessible, you need to provide a two-part text alternative.

A two-part text alternative consists of:

1. Short description: Describes the type of visual and the topic it presents.
2. Long description: Explains the essential information shown in the visual.

In the following sections, we will explain how to write effective short and long descriptions.

How to write a short description

The short description is what goes in the alt text field.

It should tell users, as succinctly as possible:

• The type of visual (chart, graph, or diagram).
• The data or topic it presents.

For example, if the image is a pie chart, the short description should mention that it is a pie chart and briefly state what data the chart shows.

How to write a long description

Depending on what the visual shows, the long description can be written as either a data table or a detailed text description.

1. Data table

If the visual presents numerical data or comparisons, a data table is often the clearest option. Tables allow screen readers to read the information in a structured way.

You can provide:

• An on-screen data table below the visual
• A link to an accessible spreadsheet that contains the same data

2. Detailed text description

If the visual communicates concepts, relationships, or steps rather than numerical data, a detailed text description may work better.

You can either:

• Provide an on-screen text description that explains all the important details, or
• Provide a link to a longer description that contains the full explanation.

On-screen description:
Place the explanation near the image, such as in a caption or a paragraph. This allows users to access the information immediately.

Linked long description:
If the explanation is too long, link to a separate page with the full description. Place the link near the image and use clear link text that indicates it leads to a detailed description of the visual.

Example: Writing short and long descriptions

Let’s look at an example to see how both descriptions work together. In this example, we will write the long description in two formats: a text description and a data table.

Short description (alt text):

Pie chart showing the distribution of monthly sales by channel.

Long description (text):

Monthly sales are distributed across five channels. Discount sales account for the largest share at 30%. Digital stores represent 27%, followed by off-line stores at 25%. Other sales make up 10%, while B2B sales account for the smallest share at 8%.

Long description (data table):

Sales Channel	Percentage of Sales
Discount	30%
Digital Store	27%
Off-line Store	25%
Other	10%
B2B	8%

Can you put the long description in alt text?

Technically, you could place the entire description in the alt text because the attribute has no strict character limit. However, this is not recommended for long explanations.

Screen readers usually treat alt text as one continuous block of content. Users cannot move forward or backward within it, or pause and resume at a specific point.

Because of this, long alt text can be difficult to process. If a user misses a detail or wants to revisit something, they have to listen to the entire description again from the beginning.

How to write alt text for line graphs

Line graphs usually show changes or trends over time.

When writing alt text for a line graph, include:

• The graph type
• What the x-axis and y-axis represent
• Important data points
• The overall trend (increase, decrease, fluctuation)
• Any peaks or drops

For line graphs, we follow the same short description and long description approach.

In the short description, mention the graph type and what the x-axis and y-axis represent. The long description then covers the remaining details, such as important data points, the overall trend, and any peaks or drops shown in the graph.

Let us consider the following example to see how this works in practice.

Short description (alt text):

Line graph showing weekly website traffic, with weeks on the x-axis and visitors on the y-axis.

Long description (text):

Traffic starts at 1,000 visitors in Week 1, increases to 1,300 in Week 2, drops to 1,100 in Week 3, rises to a peak of 1,600 in Week 4, and then declines slightly to 1,400 in Week 5.

What if the line graph has multiple lines?

Things can get more complex when a line graph contains multiple lines, with each line representing a different dataset.

In such cases, it is often helpful to provide the underlying data as a table. A data table allows screen reader users to navigate values row by row and compare datasets more easily.

However, a table alone may not communicate trends or insights clearly.

Because of this, it is best to provide:

• A data table for the raw values.
• A text description that explains key trends or patterns.

This combination ensures users can access both the exact data and the overall story the graph communicates.

How to write alt text for charts

Charts usually show comparisons or proportions.

When writing alt text for charts, include:

• The chart type
• What the chart is about
• The categories shown
• The value for each category

For charts, we follow the same short description and long description approach.

In the short description, mention the chart type and the topic of the chart. The long description then includes the categories and their values, allowing users to understand the data presented.

Let us consider the following example to see how this works in practice.

Example: Pie chart

Short description (alt text):

Pie chart showing smartphone market share by brand.

Long description (text):

Apple accounts for 40% of the market share, followed by Samsung with 30%, Xiaomi with 20%, and Other brands with 10%.

Example: Bar chart

Short description (alt text):

Bar chart showing number of girls and boys in four student groups.

Long description (text):

Group A has 10 girls and 20 boys.
Group B has 20 girls and 30 boys.
Group C has 20 girls and 25 boys.
Group D has 40 girls and 30 boys.

How to write alt text for flowcharts

Flowcharts illustrate a process, workflow, or decision path. The goal of the description is to explain the order of steps and the logic connecting them.

When writing alt text for a flowchart, include:

• The diagram type (flowchart)
• What process the flowchart represents
• The sequence of steps
• Decision points and their outcomes
• Where the process ends

Flowcharts follow the same short and long description structure.

In the short description, identify the visual as a flowchart and state what process it represents. The long description should then list the steps in order and describe the decisions and paths between them.

How to describe it:

• Use clear, natural sentences rather than repeating box labels
• Focus on the logic and order of the steps so screen reader users can follow the process easily

Let us consider the following example.

Short description (alt text):

Flowchart showing troubleshooting steps for a lamp that does not work.

Long description (text):

First, check whether the lamp is plugged in. If not, plug in the lamp. If it is plugged in, check whether the bulb is burned out. If the bulb is burned out, replace the bulb. If not, the problem is with the lamp itself, so repair the lamp.

How to write alt text for scatter plots

Scatter plots show the relationship between two variables.

When writing alt text for a scatter plot, include:

• The chart type
• What the x-axis and y-axis represent
• The range of values on each axis
• The distribution of points
• Any clusters, gaps, or visible trends

For scatter plots, apply the same short description and long description approach.

In the short description, mention the chart type and what the two axes represent. The long description should then explain how the points are distributed across the chart, including any clusters, ranges, or patterns.

Let us consider the following example.

Short description (alt text):

Scatter plot showing relationship between temperature and ice cream sales, with temperature in Fahrenheit on the x-axis and sales in dollars on the y-axis.

Long description (text):

Temperatures range from about 54°F to 80°F, while ice cream sales range from roughly $120 to $620. Points are spread across the chart, with lower sales appearing at cooler temperatures around 55°F–60°F and higher sales appearing at warmer temperatures above 70°F. The points generally move upward as temperature increases, indicating that ice cream sales tend to rise as the weather gets warmer.

How to write alt text for diagrams

Diagrams are used to show structures, relationships, or labeled parts of a concept.

When writing alt text for a diagram, include:

• The diagram type
• What the diagram represents
• The main parts or components
• Labels and measurements shown
• How the parts relate to each other

For diagrams, follow the same short description and long description structure.

In the short description, briefly state the diagram type and its subject. The long description should then explain the components, labels, and relationships between them.

Let us consider the following example.

Short description (alt text):

Diagram showing the stages of the water cycle: evaporation, condensation, and precipitation.

Long description (text):

Heat from the sun causes water from a body of water to evaporate and rise into the air. This phenomenon is called evaporation. The water vapor cools and forms clouds; this process is called condensation. Water then falls from the clouds to the land as rain, which is known as precipitation. The water returns to the ground and eventually flows back to the water body, continuing the cycle.

Tools to write alt text for charts, graphs, and diagrams

Writing alt text for complex visuals can take time, especially when charts or diagrams contain multiple values, labels, and relationships. Several tools can help you generate or structure both short and long descriptions more efficiently.

These tools should be used as assistive aids, not replacements for human judgment. Always review the generated text to ensure it accurately reflects the visual.

1. Dedicated alt text tools

Dedicated alt text tools are designed specifically to generate alt text for images. These tools use AI to analyze an image and suggest a description.

One example is AltTextGenerator.ai.

With this tool, you can upload an image, and it will generate alt text automatically. The tool also supports bulk generation, allowing you to scan a website or media library, detect images without alt text, and generate descriptions for many images at once.

This can be especially useful for sites with large image libraries.

While these tools can speed up the process, the generated descriptions should always be reviewed and edited to ensure they accurately reflect the information in the visual.

2. AI tools

AI assistants can also help generate descriptions for charts, graphs, and diagrams. Chatbots like ChatGPT, Claude, and Gemini can be used to analyze an uploaded image and generate draft short and long descriptions.

You can upload the visual and prompt the tool to:

Analyze this image of a chart, graph, or diagram and write accessible descriptions for it. Provide one short description (alt text) in a single concise sentence that states the visual type and what it shows. Then provide a long description that clearly explains the key information, such as axes, categories, values, trends, steps, or relationships, so someone who cannot see the image can understand the same information. Keep the language clear, factual, and avoid repeating the short description.

However, always review and edit the output to ensure the description accurately reflects the information in the visual and fits the context of the page.

Wrapping Up

Writing alt text for charts, graphs, and diagrams can feel challenging at first, but with the right approach and the right tools, it becomes much easier.

Using the techniques and tools discussed in this article, you can create descriptions that clearly communicate the data, relationships, or processes shown in complex visuals, helping screen reader users access the same information as everyone else.