Added bokeh VectorField plot

[philippjfr]

This completes compatibility for all chart plots, that only leaves the Arrow annotation and 3D Elements.

[jlstevens]

Fantastic!

Would it be possible to update the website so I can have a look at what the new bokeh plot looks like? I'm still seeing the old version:

I can merge before then if you need it.

[philippjfr]

You could trigger a rebuild, but for simplicities sake, here's a side-by-side:

I am cheating a little bit because I've not found a way to add arrows to the vectors, so really it's showing orientation rather than direction. Better than nothing though, unless you think that's a dealbreaker.

[jlstevens]

Personally, I think the direction (and not just the angle) is a pretty important part of the semantics of 'vector'. I'm now undecided, @jbednar you have lots of experience with both orientation/vector fields..what do you think?

[jbednar]

I think it's better to have this than not to, so I'm happy to see it merged. But yes, I think having some way to figure out which end is which is a dealbreaker for the majority of uses of this plot. Can you file an issue with Bokeh about how to do the arrows, both in this case and for the Arrow annotation? I'm not sure what the underlying problem is.

[philippjfr]

both in this case and for the Arrow annotation?

I'll try some ideas I have on this implementation, but bokeh's Arrow annotation is perfectly well defined, the issue is that our Arrow Element is very limited in what it can do and should be changed, see #34.

[jlstevens]

... is that our Arrow Element is very limited in what it can do

This is mainly because the holoviews Arrow was only designed as an annotation element i.e to point on things on a plot with a label. I don't think the Arrow element should be used to define vector fields, but it would be ok if the bokeh VectorFieldPlot class uses bokeh arrow annotations.

[philippjfr]

... is that our Arrow Element is very limited in what it can do

Sorry wasn't clear, what I meant is that our arrow element only has an x, y location and a direction but what we really need is for it to specify the start and end points of the arrow so you can have arbitrary directions. Nothing to do with vector fields.

[philippjfr]

Now with arrowheads:

[jlstevens]

Great!

Looks like it was a bit more effort to get the arrows positioned properly (using 'segment' instead of 'ray') but I do think supporting arrows properly is worth it.

[philippjfr]

Ready to merge.

[jlstevens]

Tests are green. I'm glad I can merge it with proper arrow support - thanks!

[jbednar]

Excellent! Thanks for doing this!

[philippjfr]

Now Arrow is really bugging me though:

[jbednar]

It does stand out there, doesn't it? :-)

[jbednar]

Probably ought to move Chart3D to the end of that list, btw; it's not the first thing people need, out of those items, and it's demotivating to see something unsupported as anything but the last thing in a list.

[philippjfr]

Agreed, although I just updated the image, I'd left out all the charts.

[jbednar]

Right; I figured that out and updated my own comment to match. :-) 3D elements are definitely their own separate thing, not being easily overlaid like the rest are, so it makes sense in many ways to have them at the end.

[philippjfr]

3D elements are definitely their own separate thing, not being easily overlaid like the rest are, so it makes sense in many ways to have them at the end.

3D Elements are overlayable with other 3D Elements, and at least in matplotlib even 2D Elements, where the z-coordinate defaults to 0.

[jlstevens]

Probably ought to move Chart3D to the end of that list

I concur.

[jbednar]

Being able to overlay 2D Elements is cool; that should be mentioned at the start of the Charts3D section.