Idea: Support legends that work for colour deficient people

[sirephil]

While it is generally OK to use continuous gradients for area-based visializations (such as heat maps), especially when graduating by lightness only, it is certainly the case that vsualisations relying on colour only to distinguish discreet values do not work for colour deficient users.

Examples of these are pie charts, stacked bar charts and multi-line graphs.

In such cases, the visualisations need to either:

1. Directly connect the legend label to the visualisation component (e.g. Place label above, next to or on/in the relevant component) when appearing just once
2. Use a secondary distinction, such as line style, area pattern or texture depending on the type of visualisation.

[clhenrick]

Thanks @sirephil for the suggestion. I can see how adding a pattern fill as an option or in addition to color would be a helpful a11y improvement.