Plotly.js: Axis matching limitation

[nicolaskruchten]

This is a pretty problematic case for Plotly.py: https://codepen.io/nicolaskruchten/pen/ExaOMRr?editable=true

In this case, all axes have matches set to the first axis (x or y) but those axes have no traces, so no information is being shared among the axes that do have traces.

I'd be tempted to call this a bug... thoughts @alexcjohnson @archmoj ?

This is a problem for Plotly.py because make_subplots creates subplots and sets matches before knowing which subplots have traces.

[alexcjohnson]

The issue is that xaxis and yaxis are included in layout but don't make it into fullLayout here, so they're not currently eligible to be used as matches='x' or 'y'. After some discussion, it seems that propagating them to fullLayout based on the fact that they're referenced by other _axes_ - despite having no _traces_ using them - is the preferred solution.

We also discussed matches='whatever' making a "match group," but this is problematic in that there may be settings in the lost axes (explicit range, for example) that would be lost using a match group solution.

A related question - though this part would clearly need to be a new feature, whereas the above may be considered a bug - is whether the trace-less subplots should show up in this case. One could argue that since xaxis.anchor = 'y' (and yaxis.anchor = 'x' but one seems like it should suffice) you should be able to at least opt in to showing the blank xy subplot.

[nicolaskruchten]

@archmoj can you try the "add it to fullLayout" solution described above please, and let us know if any existing tests break or if from inspecting the code anything seems like it will break?

[archmoj]

@nicolaskruchten sure.

[archmoj]

Minimal bug demo.

[nicolaskruchten]

I don't think this is the right bug... there is no matches in your pen?

[nicolaskruchten]

the problem in my pen is that panning any subplot should pan all of them due to matches

[nicolaskruchten]

A minimal test case would look like:

• 3 subplots: same y but x, x2, x3
• x2 and x3 have traces and matches x
• x2 and x3 should pan together even when there are no traces on x

[alexcjohnson]

Yeah sorry, I muddied the waters with my "related question" about showing subplots with no traces. There may be another variant of the bug here too though: if you anchor an axis that is shown to another axis that isn't shown, will the one that is shown end up drawing its ticks etc in the wrong place? But let's keep that separate and just look at matches for now. I suspect this anchor variant could be more contentious, as it might have to convert the visible axis to free since it has no subplot to be drawn on.

[archmoj]

Revised minimal bug demo.

[nicolaskruchten]

I don't mind the current "no traces, no subplot shown" behaviour, so yes, let's factor that out of the discussion for now.

The core issue for me is that make_subplots in Python assumes that x2 and x3 matching x will link them together even if x has no traces.

[nicolaskruchten]

Yes, the new minimal bug demo captures my intent here, thanks @archmoj :)

[archmoj]

Not clear to me, if we should still pan together if the axes matches x2 and y2 references instead of x and y?
Here is the demo.

[nicolaskruchten]

I don't understand that last question... can you rephrase?

[nicolaskruchten]

To me, if A and B match C, they should pan together, regardless if C has traces, or if C's number is greater than or less than or in between A and B's numbers :)

[archmoj]

In the minimal bug codepen, here I simply renamed x to x2 and y to y2 and used matching references to x2 and y2 instead of x and y. Do we still expect the axes to pan together, or x and y should be treated differently?

[nicolaskruchten]

Yes, see above re A,B,C ... IMO there should be no special treatment for x/y per se as opposed to x2/y2 or whatever

[archmoj]

Pasting comments from plotly_js channel here:

On Jan. 21

Mojtaba  9:43 AM

Linking subplots to the currently invalid axes appeared to be a pretty complicated process (with missing extremes to compute autorange, etc)
So I was not able to find a simple fix that we could release in a patch yesterday.
I suggest we possibly wait for Etienne to help with that.

Nicolas  10:07 AM

ok thanks for looking into it
did you consider my idea of substituting x2 for x etc?
(internally)
basically just interpreting x2 -> x <- x3 as x3 -> x2 if x is "invalid" ?

Mojtaba  10:19 AM

Yeah, I think that's the cleanest solution namely if we prefer not to display the x.y sub-plot in future.
Let me give that a second try.

[etpinard]

Ok, let me try to summarise the current state of affairs.

---

Minimal example: https://codepen.io/MojtabaSamimi/pen/wvBRGyV?editors=0010

where xaxis2, xaxis3 and xaxis4 all have matches: 'x' which is currently invalid i.e. it isn't part of this list:

https://github.com/plotly/plotly.js/blob/8122c2127c12b50cb20207d676aa24e3a786f305/src/plots/cartesian/layout_defaults.js#L151

Proposed solution 1

From @alexcjohnson 's https://github.com/plotly/plotly.js/issues/4501#issuecomment-576498852

After some discussion, it seems that propagating them to fullLayout based on the fact that they're referenced by other axes - despite having no traces using them - is the preferred solution.

@archmoj attempted this in https://github.com/plotly/plotly.js/commit/137d73d29669acdcf0c02b156d572f214fcb946c, but ran into problems trying to make this solution work, saying: "appeared to be a pretty complicated process (with missing extremes to compute autorange, etc)"

Proposed solution 2

From @nicolaskruchten 's https://plotly.slack.com/archives/C07MRPNHW/p1579705647015700

did you consider my idea of substituting x2 for x etc? basically just interpreting x2 -> x <- x3 as x3 -> x2 if x is "invalid" ?

This one is in PR https://github.com/plotly/plotly.js/pull/4506

As pointed out in https://github.com/plotly/plotly.js/pull/4506#issuecomment-577701392,

One thing this approach misses is if the lost axis sets an explicit range or rangemode https://codepen.io/alexcjohnson/pen/wvBNXgY?editors=0010
That seems like something we want to support, right?

and in https://github.com/plotly/plotly.js/pull/4506#issuecomment-577715994

Also categoryarray and any other axis-related option I would guess. That is a drawback to this approach: you need to copy everything over basically

---

To me, solution 1 appears more robust, but also more complicated to implement.

But, it sounds like we could get solution 2 to work by finding the set of attributes that we need to "copy over" and by finding a way to remind us to potentially add attributes to that set whenever we add new cartesian axis features.

@archmoj would it be ok if I take a shot at implementing solution 1 and see just how "complicated" it gets?

[archmoj]

@archmoj would it be ok if I take a shot at implementing solution 1 and see just how "complicated" it gets?

@etpinard Sure. Please go for it.

[archmoj]

@etpinard
My dev branch is at: https://github.com/plotly/plotly.js/commits/fix4501-match-axes_dev