zeroline is not visible when axis has no gridlines

[DaanyWaay]

https://codepen.io/anon/pen/pXxqeW
zeroline is not visible if these conditions are met:

rangemode: 'tozero',
nticks: 1 (or 2)

If you change nticks to: 0 | 3 | 3+ it is shown again.
Unfortunately I need to hide all the ticks except tick0 as this configuration will be used for subplots.

The solution with mirror axis works better, but:

• line between subplots has 2px
• the tick is not directly under zeroline
  https://codepen.io/anon/pen/OeBGjN

[etpinard]

Thanks for writing in!

At present, we don't draw the ax.zeroline line on axes that don't have any visible grid lines associated with them e.g. as in the nticks: 1 or nticks: 2 cases you pointed out (note also that 0 isn't a valid nticks values, so you get the default ticks in that case).

The logic is here:

plotly.js/src/plots/cartesian/axes.js

Lines 2732 to 2745 in 323e79a

	axes.shouldShowZeroLine = function(gd, ax, counterAxis) {
	var rng = Lib.simpleMap(ax.range, ax.r2l);
	return (
	(rng[0] * rng[1] <= 0) &&
	ax.zeroline &&
	(ax.type === 'linear' || ax.type === '-') &&
	ax._gridVals.length &&
	(
	clipEnds(ax, 0) ||
	!anyCounterAxLineAtZero(gd, ax, counterAxis, rng) ||
	hasBarsOrFill(gd, ax)
	)
	);
	};

---

I can't remember exactly why the ax._gridVals.length condition exists. We may consider changing it and honouring ax.zeroline: true no matter what, but that would have to be part of a major version update (e.g. in our v2 release).

[etpinard]

On second thoughts, I'm starting to think this report should be considered a bug (thanks @alexcjohnson )

Removing

plotly.js/src/plots/cartesian/axes.js

Line 2738 in 323e79a

ax._gridVals.length &&

does not make any of our tests fail.

The only problem I see is: as some axes coerce('zeroline', true), according to the logic below

plotly.js/src/plots/cartesian/line_grid_defaults.js

Line 56 in 674f0b0

var showZeroLine = coerce('zeroline', opts.showGrid || !!zeroLineColor || !!zeroLineWidth);

a "new" zeroline will appear on some graphs. Now I agree that:

xaxis: {
  zeroline: true,
  nticks: 1
}

should show the zeroline, even when it's on one of the plot area edges.

Would anyone be opposed to adding a ax._input.zeroline condition in here:

 axes.shouldShowZeroLine = function(gd, ax, counterAxis) { 
     var rng = Lib.simpleMap(ax.range, ax.r2l); 
     return ( 
         (rng[0] * rng[1] <= 0) && 
         ax.zeroline && 
         (ax.type === 'linear' || ax.type === '-') && 
         (ax._gridVals.length || ax._input.zeroline) &&
         ( 
             clipEnds(ax, 0) || 
             !anyCounterAxLineAtZero(gd, ax, counterAxis, rng) || 
             hasBarsOrFill(gd, ax) 
         ) 
     ); 
 }; 

[alexcjohnson]

My gut reaction is if zeroline gets coerced true, we should show it in these edge cases too, not just when it’s explicitly set true. Is there a case where that would look wrong?

[etpinard]

I'm going to slip in a fix for this bug i.e. by simply removing

plotly.js/src/plots/cartesian/axes.js

Line 2738 in 323e79a

ax._gridVals.length &&

in 1.50.0.