Hands-on session 1: Writing alt-text for biomedical data visualization figures

Alt text and figure captions for manuscripts

Research benefits.

  • Accessible manuscripts ensure the most people can engage with your work.
  • Accessible manuscripts are easier to consume, quote, cite, share, and discuss.

To improve manuscript accessibility:

  • Reduce language and cultural barriers through plain language and translation.
  • Reduce financial barriers through open access publishing.
  • Reduce perceptual and cognitive barriers through high-contrast text, appropriate font sizes and clear image resolution, as well as alt text and figure captions.

Accessibility improves usability for a broader audience, including people with disabilities, non-native speakers, and those without institutional access. This expands readership, citation potential, and collaborative value.


Alt text and figure captions:

Essential for some. Useful for all.


Alt text vs figure caption

Disambiguation


Alt text

Answers “what is this image?”

  • Depends on the image’s placement and context within the document
  • 1-2 sentences.
  • An alternative to the image.
    • Essential for blind or low-vision users to know what an image is for when using screen readers.
    • Useful for all when an image can’t load

Alt text summarizea an image with regards to its role within the context of the document.

Alt text does not exist in a vacuum. The same image can have different alt texts, depending on where it is used, when it is used, and for what purpose.


Alt text for diagrams and data visualiations

Best practices:

  • Summarize the key meaning and data points in a sentence or two.
  • For complex images:
    • Consider supplementing with adjacent page text.
      • Add a figure caption.
      • Add description in article content.
    • Consider providing a link the the full data.

Context matters: The key meaning and data points depend on the reason for including the diagram and data visualizations within the narrative of the manuscript.


Figure caption

Answers “what is this figure telling me?”

  • Accompanies an image.
  • Can be longer than 1-2 sentences if needed to fully explain the figure.
  • Informed by the author’s purpose of including the figure in the document

  • Essential for all to understand a figure’s purpose, especially when it’s complex or includes data.
  • Useful for all when viewing a figure is difficult or inconvenient; such as on a small screen, in low bandwidth settings, or when reading via text-to-speech.

Everyone sees the figure caption – sighted users, screen reader users, etc.

It points out what’s significant about the image, calling out any points relevant to your document.

Readers should be able to read the caption to understand what the image is saying.


Example

Zero context = ambiguous alt text

image of a genomics visualization

Without context, this image’s alt text and caption can be ambiguous.

Within the context of this presentation, its alt text is an “image of a genomics visualization.”

Within the context of this presentation, its caption might be “Figure 1: a genomics image being used for example purposes.”


More context = less ambiguity

Screenshot of the heading of the Nature Genetics article titled "A single-cell transcriptome atlas of human euploid and aneuploid blastocysts"

Within the context of a Nature Genetics manuscript titled “A single-cell transcriptome atlas of human euploid and aneuploid blastocysts,” this same image would likely have a different alt text. If it is a figure, it might even have a figure caption.


The reality: The image is the first figure in the manuscript, appearing in the results section. It is referenced in the first paragraph of the results section.

More context = less ambiguity.

The image alt text and caption should take this context into consideration.

Assume a broken image in this context: /assets/Screenshot 2025-06-05 at 1.39.02 PM.png size: contain

What should be its alt text and figure caption?


What did Nature Genetics do?

Alt text: “Figure 1.”

  • Not ideal.
  • Screen readers will read “Figure 1,” but the user doesn’t know what the figure is, just its label.
  • If the image doesn’t load, users receive no meaningful content.
  • Better: “Diagram showing results of single-cell transcriptome profiling.”

Figure caption: “Fig. 1: Single-cell transcriptome profiling delineates the emergence of four lineages in human aneuploid blastocysts.”

  • Adequate.