Allow casting from GridImage to QuadMesh

[philippjfr]

Useful now and will continue working just fine in the future when the image interface has been added. Lets you do something like this to get hover information from a datashaded plot:

%%opts QuadMesh (fill_alpha=0) [tools=['hover']]
points = hv.Points(np.random.multivariate_normal((0,0), [[0.1, 0.1], [0.1, 1.0]], (1000000,)))
datashade(points, width=400, height=400) * hv.util.Dynamic(aggregate(points, width=100, height=100),
                                                           operation=lambda x: hv.QuadMesh(x))

[jbednar]

Looks good to me and seems safe; happy to merge when tests pass.

[jlstevens]

Looks good to me as well. Merging.

[stephanecollot]

That looks great!

Unfortunately I didn't succeed to make it work in my notebook. Could you show us the imports that are loading 'datashade' and 'aggregate'?
Also, is it possible to make it work for multiple time series?

Thanks for your help

[jbednar]

You can see how to use it in https://anaconda.org/jbednar/holoviews_datashader , which shows all required imports, and also shows how to work with multiple plots. This all requires conda install -c ioam holoviews.