GUI: add divergent colour scheme

[shawnlaffan]

Many indices follow a symmetrical distribution around some value. The main example is the z-scores.

It would be useful to plot these using a divergent colour scheme such as in the colorbrewer set.
https://colorbrewer2.org/#type=diverging&scheme=RdYlBu&n=3

This can be user-selected with the Hue, Sat and Grey options, or it can be automatic (possibly also user controlled).

The automated option can build on some of the recent changes in #819

[shawnlaffan]

In the end there were not that many indices so it is done as an automated process.

This issue will be closed once the blog post is out.