It is read by screen readers in place of images allowing the content and function of the image to be accessible to those with visual or certain cognitive disabilities.
It is displayed in place of the image in browsers if the image file is not loaded or when the user has chosen not to view images.
It provides a semantic meaning and description to images which can be read by search engines or be used to later determine the content of the image from page context alone.
Alt and surrounding text
"CHART TYPE of TYPE OF DATA where REASON FOR INCLUDING CHART`
+ Link to data source somewhere in the text
CHART TYPE: It’s helpful for people with partial sight to know what chart type it is and gives context for understanding the rest of the visual.
TYPE OF DATA: What data is included in the chart? The x and y axis labels may help you figure this out.
REASON FOR INCLUDING CHART: Think about why you’re including this visual. What does it show that’s meaningful. There should be a point to every visual and you should tell people what to look for.
Link to data source: Don’t include this in your alt text, but it should be included somewhere in the surrounding text.
Accessible Visualization via Natural Language Descriptions: A Four-Level Model of Semantic Content
Alan Lundgard, MIT CSAIL
Arvind Satyanarayan, MIT CSAIL
IEEE Transactions on Visualization & Computer Graphics (Proceedings of IEEE VIS), 2021
To demonstrate how our model can be applied to evaluate the effectiveness of visualization descriptions, we conduct a mixed-methods evaluation with 30 blind and 90 sighted readers, and find that these reader groups differ significantly on which semantic content they rank as most useful. Together, our model and findings suggest that access to meaningful information is strongly reader-specific, and that research in automatic visualization captioning should orient toward descriptions that more richly communicate overall trends and statistics, sensitive to reader preferences.