Hover styling for dataset in 'dataset' mode

[kurkle]

Dataset level hover styles for dataset mode.

Pen

Closes: #2136
Partially solves: #3880 (hover only triggered by points still, not filled area as requested)

[benmccann]

I just had an idea. What if we call _resolveDatasetElementOptions again, but with a new argument denoting that the dataset is currently being hovered? Then we would support all chart types instead of just line and it would be scriptable by the user.

We could automatically look for options of the same name prefixed with hover and give them priority. We could also add hover: true to the context so that it can be scriptable by the user

[benmccann]

I think it'd be nice to update setHoverStyle to use _resolveDataElementOptions similar to the way setDatasetHoverStyle works

I also think some of the other docs need to be updated as well like bar chart now that this is implemented generically. Maybe we can document hover generically?

[kurkle]

I think it'd be nice to update setHoverStyle to use _resolveDataElementOptions similar to the way setDatasetHoverStyle works

Agreed, but its not quite in the scope of this PR

I also think some of the other docs need to be updated as well like bar chart now that this is implemented generically. Maybe we can document hover generically?

Bar does not utilize dataset element, so this does not apply there. Not quite sure how to document hover generically.

[benmccann]

Should we document hoverCubicInterpolationMode and hoverFill?

[kurkle]

Neither of those work. We are not going through update on hover, just updating the model with options and then render.
So bezier control points don't get updated.
I did not investigate why hoverFill does not work.