Write alt text for vignette figures

[lambdamoses]

At least for the Visium vignettes but ideally all vignettes. It's a lot of work since there are so many vignettes. Ideally there should be a way to sonify spatial data or convert to tactile.

Alt text resources:

• Web Accessibility Initiative's Alt Text Decision Tree
• Web Accessibility Initiative's Complex Images page
• Open Educational Resources (OER) Accessibility Toolkit
  - Scroll down to find 'Best Practices 2: Images' and 'Best Practices 3: Tables'

Captions are best when you're describing complex images (like diagrams or graphs) because captions can contain a lot more information. Alt text is usually brief.