Data Visualization: Alternative Text in Data Visualization

What is alt text?

Alt text, short for alternative text, is a textual description provided for non-text content, such as images, graphics, or diagrams. It serves as a substitute for the visual information that these elements convey.

Why is alt text important for accessibility?

Alt text provides individuals who are visually impaired or using assistive technologies, such as screen readers a textual representation of the image. By describing the image accurately, alt text enables these users to understand the purpose and meaning of the image within the context of the surrounding content.

Figure 1. Example for charts courtesy of U.S. General Services Administration.

Descriptive alt text: "Bar chart of event sales for Monday through Friday. Amounts for ticket, food, and merchandise sales show a slight decline from Monday to Wednesday, with a dramatic increase to peak on Friday. Table 1 further details the event sales by category."

Vague alt text: "Event sale graph"

View more graph examples in Authoring Meaningful Alternative Text from U.S.G.S.A.

Tips for writing alt text:

• Be concise - Include only essential details.
• Avoid "image of" or "picture of". Screen readers announce this automatically.
• End with a period. This ensures a brief pause for screen readers.
• Describe the image's purpose. Focus on what the user needs to know, not just what's visible.
• Include text from images. If an image contains important text, add it in the alt text.