Testing the bokeh Elements tutorial and matching the original

I think it would be great to just copy the current Elements tutorial, set hv.notebook_extension(bokeh=True) at the top and make it available on the website (the tutorials could then be called Elements [matplotlib] and Elements [bokeh]. This would achieve a few things:

1. Show some official bokeh examples on or website.
2. Show how easy it is to use the same code and switch backends.
3. Test our bokeh support.
4. Show people the full range of things that can be done with the bokeh backend.

The main issues I can see are:

1. Setting this up on Travis (I think bokeh is already installed though).
2. Getting display tests working for bokeh. This may be tricky if there is a fair bit of randomness (e.g uuids) in the bokeh output given the same input.

[jbednar]

Right; as discussed in https://github.com/ioam/holoviews/pull/1196 that’s the only non-3D Element not yet supported by the Bokeh backend.

[jlstevens]

The idea of setting up fallbacks in this way is one Philipp and I have toyed with but we aren’t completely convinced this is exactly the behavior we want. As it would also require some refactoring, this suggestion is something to think about after 1.4.1 is released.

For now what I have done (PR #374) is update notebook_extension to activate the last backend loaded in the resource list. As Jim noted, we can’t know the specified order left-to-right using the keyword syntax:

notebook_extension(bokeh=True, matplotlib=True)

But now you can pass kwargs which is shorter and preserves the order:

notebook_extension('bokeh', 'matplotlib')

In this case, BokehJS would be loaded and the matplotlib backend would remain active. Specified the other way round and the bokeh backend would be automatically activated.