Add layout.subplots to enable (x|y) hover effects across multiple cartesian and splom suplots sharing one axis

[archmoj]

Resolves #4755 and resolves #2114.

@plotly/plotly_js

TODO:

• Handle splom
• Should it be new hovermode options e.g. 'x all', x all unified, y all and y all unified instead? --> No.
• Improve positioning of unified hover to be closer to initial hovered subplot

[emilykl]

LGTM @archmoj .

Re: API design -- I do think this should be its own property, the way you have it, rather than adding more hovermodes, since adding so many keywords in the hovermode starts to get pretty unwieldy. I'm not sure the name hoversameaxis captures what this feature actually does though. Thinking on it.

Maybe something like hoverthrough?

[archmoj]

LGTM @archmoj .

Re: API design -- I do think this should be its own property, the way you have it, rather than adding more hovermodes, since adding so many keywords in the hovermode starts to get pretty unwieldy. I'm not sure the name hoversameaxis captures what this feature actually does though. Thinking on it.

Maybe something like hoverthrough?

Thanks @emilykl for the feedback.
I was thinking of a separate case that we may want to support in future i.e. something like hovermode: 'xy' or '+' it could display effect on subplots on both x and y directions.

Also it may be possible to add another option to this feature so that the hover show points in suplots above & below (or left & right) even if they are NOT linked to the same axis.

Perhaps instead of the *true* option we should have a string option so that it could be developed further.
Something like: hoverthrough: 'axis'?

cc: @alexcjohnson

[alexcjohnson]

How about layout.hoversubplots with values (eventually, wouldn’t need to implement all of these now) single (just the axis pair of the primary point), overlaying (all subplots using the main axis and occupying the same space), axis (also include stacked subplots using the same axis) and matching (also include other axes matching the main axis)

[emilykl]

How about layout.hoversubplots

I like this.

[alexcjohnson]

Splom has an additional constraint that I don't believe is tested here, and I suspect is broken: If the main hovered point has the same coordinate as some other points, every point we show on other axes should be the same item on other axes. For example, let's say we have this data:

dimensions: [{
  values: [1, 1, 1, 1]  // hover on a subplot where this is the x axis, so all points are in a vertical line
}, {
  values: [1, 2, 3, 4]  // put the mouse at y=2 on this subplot
}, {
  values: [1, 2, 3, 4]  // we should see y=2 hovered in this subplot as well
}, {
  values: [1, null, 3, 4]  // no hover points should be displayed on this subplot
}]

[alexcjohnson]

This actually makes me wonder if we should use pointNumber as a tiebreaker (and possible performance improvement) for generic multi-trace hover, either same or different subplots: After we've found the primary hovered point, for each additional trace we examine, see if (a) the data arrays have the same length as those for the primary point, (b) the data point with the same index satisfies the criteria to be another point in the hover set (ie the x value matches, for x or x unified modes). If so, use it and don't bother searching the whole trace. If either of those conditions is not met, assume the points in these traces aren't supposed to correspond 1:1 to each other and continue with the way we do this today.

[archmoj]

Good catch for splom.
It is addressed in d9aea4b.

[alexcjohnson]

💃 lovely!

[wuyuanyi135]

would typescript stubs get updated for this?

Current latest:

"@types/plotly.js": "^2.33.3",
"@types/plotly.js-dist-min": "^2.3.4",