Bokeh bar plot

[philippjfr]

In #1407 I outlined the way forward for the Bar plot implementations. Since bokeh charts will be split out imminently and I'll be using Bars in at least one project I thought it would be worth replacing the bokeh charts based implementation immediately. This new implementation retains all the functionality of the charts based implementation, while adding more features and improvements:

• Better control over legends
• Better support for updating barplots as part of HoloMaps/DynamicMaps
• Much better performance (at least 10x faster for more complex charts) because this implementation uses one vectorized glyph rather than a bunch of individual glyphs like the chart based implementation did.
• Support for colormapping bars
• Better support for overlaying bars
• Support for horizontal bars via the invert_axes plot option
• Support for log mapping bar heights

Here is the notebook I used to develop the plotting class with an extensive list of examples.

[philippjfr]

I also just added support for batching BarPlots, this means that you can stack and group NdOverlays of Bars, which means you can use the simpler Bars format with only a single key dimension. In other words these two versions are equivalent:

%%opts Bars [stack_index='group]
hv.Bars((range(10)*5, sorted(map(chr, range(65,70))*10), np.random.rand(50)), kdims=['x', 'group'])

%%opts NdOverlay [legend_limit=0] Bars [stack_index='group' show_legend=True]
hv.NdOverlay({chr(65+i): hv.Bars(np.random.rand(10)) for i in range(5)}, kdims=['group'])

[philippjfr]

Ready to merge once tests are passing.

[jlstevens]

The improvements list is quite impressive and we are explicitly moving away from the bokeh charts API so this work was definitely necessary.

The tests are passing and I am happy to merge. My only comment here is regarding unit tests, do you you think existing unit tests are sufficient or can we add more to test this new implementation?

[jlstevens]

I think the issue regarding additional testing can be deferred by #1422 (assigned to version 1.8). Merging.