Complex images like charts & graphs must be briefly described using basic alt text and must have a more complete long description elsewhere in the page. The long description should be available to both sighted and screen-reader users and be programmatically associated with the image.

The figure / figcaption method:

One of the limitations of basic alt text is that it’s meant to be brief, no more than around 150 characters. So you shouldn’t try to use an alt attribute in an image for the long description. Instead, a good way to add a programmatically associated long description is the use the figure / figcaption method. In this method the figure tag wraps the whole image and long description code. Then the figcaption tag is used to wrap the long description text. This can be normal HTML text that’s available to both sighted and screen reader users. Inspect the code of this preformatted example to see how the figcaption tag is used house a long description.

The aria-describedby method:

You can also use the aria-describedby attribute added to an image to point to the id of a long description. For example, you could point to the id of a div wrapping a long description like:

    

    

    
Factors in Your Credit Score
    

    

    

     
Payment History (35%)
    
35%
    
    

    
Outstanding Debt (30%)
    
30%
    
    

    
Credit History (15%)
    
15%
    
    

    
Pursuit of New Credit (10%)
    
10%
    
    

    
Types of Credit in Use (10%)
    
10%
    
    
    
    

Inspect the code of this preformatted example to see how aria-describedby works. In this example, the table within the wrapper div describes the segments of the pie chart. It wouldn’t be possible to descibe all those segments in basic alt text.

The web accessibility guideline SC related complex images is 1.1.1 Non-text Content (Level A). The W3C WAI has a tutorial with more information about handling complex images. Most universities are striving to meet WCAG level AA conformance. Of the three levels of WCAG conformance, level A is the most important to achieve and level AAA is nice to have but not mandatory. So level AA is a middle ground that most universities and government agencies strive to achieve. Ohio State’s information about minimum web accessibility standards can be found in the Digital Accessibility Center’s website.