Refactored plotly backend

[philippjfr]

Cleanup and refactoring of the plotly backend.

• Adds Path3D element Inclusion of a 3d lines functionality ? #2453
• Consistently handles invert_axes and invert_x/y/zaxis options
• Adds support for xticks/yticks/zticks
• Ports basic dim transforms to plotly backend
• Updates TablePlot to trace type
• Adds LabelPlot, AreaPlot, SpreadPlot, ViolinPlot
• Adds unit tests for element plot classes
• Add documentation for plotly containers
• Added/updated element reference notebooks

[philippjfr]

@jonmmease Still working on this a bit but I wanted to check with you before getting much further into this. The PR refactors the element plotting classes a bit. The main changes is that instead of declaring the trace_type each class now declares some trace_kwargs which can include a bit more info than the type and then it refactors the get_data and graph_options methods to allow returning multiple sets of data.

[jonmmease]

@philippjfr, I did look over it briefly last night and this looks/sounds like a nice generalization that will be useful to support future elements. Thanks!

[philippjfr]

Okay, I think I'm mostly done here. I think it is indeed a nice generalization which makes writing new plotting element plotting classes a lot easier. One thing that should be generalized further is the way styling is applied, i.e. certain style options are set on the marker while others are set on the data itself and that is not nicely handled yet. However for the time being this is already much more flexible than it was. I've also finally added unit tests for all the element plots.

[philippjfr]

Ready to review and merge.

[jonmmease]

wow @philippjfr, this is awesome! The plotly backend is really coming along now! Everything looks great from my end, and I'm excited to play with it soon 🙂

[jlstevens]

Notebook metadata needs to be stripped out.

[philippjfr]

Don't see any.

[jlstevens]

I've not yet reviewed the code but I do see this PR is introducing a bunch of new notebooks.

I'll be making a PR today updating all the bokeh reference gallery notebooks to use the new options style and I don't want to merge any new notebooks unless they conform to that style as well.

Edit: The PR showing what has been updated for Bokeh is #3269

[jlstevens]

@philippjfr told me this is now ready to merge.

Thanks for updating the notebooks as requested. Merging.

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