Allow easier editing of colour scale extremes

[guygriffiths]

It seems that the colour scale extremes can only be edited by mouse, which makes it hard to set precise values. These should be editable by keyboard too. Also it would be useful to have a button to make the colour scale centred on zero.

(In fact the latter facility could be useful generally. Certain variables could be designated "zero-centred", which means that by default their colour scales are divergent. When editing one extreme of the scale, the other extreme would automatically change to the negative of the first extreme. This could perhaps be toggled on/off.)

Note: This issue has been automatically migrated from Bitbucket
Created by jonblower on 2014-08-18 08:23:58+00:00