A graphic has a significant amount of text embedded in it and is distributed as an image (ie. JPEG, PDF). When text is saved as an image that information is effectively hidden from assistive technology users.
People who are blind, people with some visual processing difficulties.
What to do
If you can summarize the content in a few sentences, use the caption. (or the alt text if no caption is available). If the description requires more than a few sentences:
- Create a text description document in Word or Google Docs
- Choose the “Publish as html” or “Publish to the web” option within that document’s File menu
- Link from your web page to the document
- Remember to add a return link at the end of your description so the user can easily get back to the original page!
You may also want to provide a link to the full data set in an accessible format (see document accessibility).
The Diagram Center has a pretty comprehensive set of guidelines for charts and graphs.