[FEATURE] Add fill properties to slope

[kylrth]

It'd be useful to have an annotation like BoxAnnotation that creates a dividing line, but which can be angled. It's possible to achieve something similar with an image, but then interaction is extremely slow as the plot has to re-render the image at every frame.

Using bands

This may be possible if we expand the features of the Band annotation. Currently, leaving lower or upper unspecified results in a blank figure, but we could change the behavior such that if either is unspecified the band continuous off infinitely.

I'd like to hear the thoughts of those more familiar with the capabilities of annotations.

[bryevdv]

I think allowing Band to continue off infinitely in one direction is a good feature, but it won't get you an infinite division, so I don't think it is this feature. One dimension of the band will always be finite (because explicit coordinates are used). In order to achieve an infinite "split" as asked about in the Discourse issue I would suggest adding two optional fill properties to Slope. The Slope annotation is naturally "infinite" already, so extending it to optionally fill either side of the infinite division would be fairly simple. Main observations:

• what is the right terminology? left_fill_color makes sense until the slope is perfectly horizontal. I could live with a documented convention (e.g. left=above) for this special case, but maybe there are better names?

• This only affords dividing the plane visually in to two pieces, not more. If there was really demand for multiple infinite bands we could add SlopeSet (to match LabelSet) but I'd want to see some sustained interest before putting work in to that.

[kylrth]

If people are looking for a vertical line, they'll choose a Span, right? Can you even specify Slope(gradient=np.inf)? I think fill_above and fill_below (or some variant thereof) will always make sense for the parameters of a Slope object.

[bryevdv]

I had forgotten Slope is configured with an intercept and derivative (instead of a point and an angle, say) so yes I agree it's not possible to make a vertical slope, and that above/below are therefore well-defined always.

[kylrth]

Add this to the list of things I will "eventually" work on. :P

[bryevdv]

Just noting that the way properties are named is slightly constrained, so for concreteness the proposal is to add above_fill_color and below_fill_color:

slope = Slope(gradient=gradient, y_intercept=y_intercept,
              line_color='orange', above_fill_color="red", below_fill_color="blue")

and similarly for above_fill_alpha and below_fill_alpha. It would be nice to also add above/below hatch properties at the same time but that's certainly a lower priority.

[graeme-hattan]

If you add above_fill_color and below_fill_color to Slope, then what is missing is the ability to fill between slopes like you can for horizontal/vertical lines using BoxAnnotation. Maybe a better solution is along the lines of adding gradient parameters to BoxAnnotation (then probably renaming the class to something more descriptive and then making BoxAnnotation a subclass which sets up horizontal/vertical borders).