Changing histogram colours should not run a new query

[avleen]

Changing the colours on a histogram should only cause the colours on the histogram to be updated.
Currently, it causes a whole new query to be run - which can be really really slow when all you want to do is change colours to make things a little clearer, or try different colour schemes.

[thomasneirynck]

perhaps same root cause as #7945. Needs to be verified if this still is an issue.