All charts and images must have alt text, or alternative text, to summarise the information presented for users who cannot see the visual. It can also be used to describe what should be on the page if the web browser fails to load the images. Screen readers read this out for people with visual disabilities.
Alt text should be around 125 characters (15 to 20 words), be no more than one sentence and include:
- the chart type used
- the type of data used in the chart
- a summary of the main trend of the chart
Avoid using a literal description of the image or chart and instead focus on what point it is making. Alt text can duplicate part of the descriptive chart title but should not repeat content in the main text.
When describing this chart, the alt text could read: Line chart of marriage rates where the amount of opposite-sex marriages peaked in 1972 and has generally fallen since.