Graphs/charts are great ways to visualize data. But, it can be difficult to figure out how to summarize a visual representation of data for alternative text. Keep the following in mind:
In your alt text, link back to the data in a separate document. Your description will not be able to capture all the information your chart represents. With the link, a user can get to the data if they want to know specifics.
To learn more about what to include in the description and where to include it, please visit Arizona State University's complex images (i.e. graphs) guidelines.