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 (for example, bar chart, line chart)
- the type of data used in the chart (for example, marriage rates, death rates, level of GDP, amount of weekly hours worked)
- 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. This causes “auditory clutter” for screen reader users where information may be repeated several times. Screen readers will read out the chart title and subtitle, so do not worry about fitting all of this information into the alt text as well.
When describing this chart, the alt text could read: Line chart showing the amount of weekly hours worked is still low but showing signs of recovery.