Avoid a diverging colour scheme where the midpoint isn't meaningful

[iaindillingham]

We should avoid using a diverging colour scheme (ColorBrewer RdBu) on the maps in OpenPrescribing, as the midpoint isn't meaningful. For example, in this map of co-codamol vs patients on list by sub-ICB locations, the smallest value (blue) represents 3.67; the largest value (red) represents 56.69; the midpoint (white), represents 26.51. Clearly, the encoded range and the data domain are incongruent. Instead, we should prefer a single hue sequential scheme, such as ColorBrewer Blues or Reds.

For more information, see the section on Colour Schemes in the Vega documentation; and the ColorBrewer project.