Plot utils

[cseed]

I'm not sure if this is worth it, but gives the plotting library a slightly more self-contained feel.

[tpoterba]

I'm fine with this and do agree it's a QoL improvement, but want to raise the point that learning Hail's plotting library is not sufficient for someone's visualization needs - a user needs to learn bokeh (or something else), or will feel extremely constrained by the narrow set of things Hail can do at the moment. I do worry that hiding bokeh in this way will preempt that learning process.

I think we should merge this, but want to hear your thoughts first.

[cseed]

I agree completely. I certainly don't think we can hide or replace Bokeh (I hope the explicit emphasis on Bokeh in the documentation makes this clear), but I think we should continue to add common-case utilities to hl.plot life easier for users (and ourselves).

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