@dkillick ..
Actually, I realised don't really like these lines, which I originally added because you suggested it might make the use of 'extend' clearer :
> If you're going to extend it would be nice to have a discrete colour for them, so that you can differentiate between elements that fall within and without of the colorbar's range...

On further reflection, I don't think I should have added this, as the results add nothing to the actual plots (some almost invisiible under-range areas and no over-range, as it happens), and I'm wondering what the point is, as this adds more visual clutter in both code and the plots.

So I'm taking it out again:  I think its ok that the extensions are just there to make it clear that any out-of-range values are coloured the same as the range extremes.