10.5 Practising Data Visualisation

Considerations for accessibility

Making something accessible means designing a product or service in such a way that people are not excluded from using it due to a disability or impairment.

Important to plan for:

  • Contrast.

  • Visual impairments (blindness, low vision, myopia, presbyopia, hypermetropia).

  • Colour vision deficiency (colour-blindness).

    • Colour blindness affects approximately 1 in 12 men (8%) and 1 in 200 women (0.5%).
    • Protanopia (red-blind): no shades of red can be seen.
    • Protanomaly (red-weak): some shades of red may be seen.
    • Deuteranopia (green-blind): no shades of green can be seen.
    • Deuteranomaly (green-weak): some shades of green may be seen.
    • Tritanopia (blue-blind): no shades of blue can be seen.
    • Tritanomaly (blue-weak): some shades of blue may be seen.
    • Monochromacy (achromatopsia): no colour, only shades of grey.

Conversely, approximately 12% of women possess “super colour vision” (tetrachromacy).

Contrast

W3C Content Accessibility Guidelines (WCAG) recommend a contrast ratio of:

  • at least 4.5 for regular text on a background colour;

  • at least 3 for large text, graphics and UI components (e.g., form input borders).

Contrast ratio is defined by W3C as L1+0.05L2+0.05, where L1 and L2 are relative luminance (brightness) of lighter and darker colours.

For sRGB, relative luminance is L=(0.2126×R)+(0.7152×G)+(0.0722×B)

Useful packages to try:

Vision impairments

Alternative text

A excellent blog post has been written by Amy Casel who provides the following template: alt = “[chart type] of [data type(s)] where [reason for including chart].”

Alt text can be added to code chunks, within the chunk parameters, with alt="description of chart".

Accessible data tables

  • Provide alternative formats for datasets, such as a data table in CSV or ODS format.

  • The table should be made with correct markup for headings and column names.

Examples of how to structure accessible tables can be found in the Guidance Hub of the UK Government Analysis Function, Data visualisation: tables.