Bokeh adjoined histogram adds support for live colormapping

[philippjfr]

An adjoined histogram in bokeh now automatically shares the color mapper of whatever plot it is adjoined to it and adds a box_select tool with a CustomJS to update that colormapper based on the current selection. Since this callback is pure JS this will also work on static export.

[jbednar]

Cool! The code looks clean to me, but maybe better if @jlstevens merges it just to be sure.

[philippjfr]

Based off #927, so that should be merged first.

[jbednar]

#927 is merged, so can this be merged now?

[philippjfr]

Yes, I just have to rebuild test data first (because you broke it 😄).

[jbednar]

Is the test data in github? Seems easier just to edit that one character.

[jlstevens]

PR #927 has now been merged and this functionality is certainly very nice (especially as it doesn't need a live server). My only concern is that it doesn't seem hugely intuitive to me to use the box select tool on a side histogram. Other than that, I am happy to merge.

[jbednar]

The box select tool seems very intuitive to me, in terms of mouse clicking; it's just that ideally the visual representation of the selection would not be a box but a range of histogram bins. I.e., the same clicks should do the same thing, but the selected area should show bins, not portions of bins, ideally. Still, that seems like a nicety that could be added someday, and shouldn't hold up this PR.

[jlstevens]

It is certainly nice functionality! Merging.