Allow fixedrange axes to scaleanchor with constrain:domain

[etpinard]

Compare

• https://codepen.io/etpinard/pen/daPyXd
• https://codepen.io/etpinard/pen/MLYWjy

where the first one has fixedrange:true on both x and y axes, and the second does not. This was first reported in https://community.plot.ly/t/axis-constraints-without-zoom-use-scaleanchor-with-fixedrange/18354

Now, the scaleanchor logic is skipped for fixedrange:true. This makes sense when constraining by range (from #1522), but does it also make sense when constraining by domain (from #1767)?

Commit dee136d was added in the first scaleanchor PR #1522, to:

plotly.js/src/plots/cartesian/constraint_defaults.js

Lines 58 to 61 in e4e1149

	Lib.warn('ignored ' + containerOut._name + '.scaleanchor: "' +
	containerIn.scaleanchor + '" to avoid either an infinite loop ' +
	'and possibly inconsistent scaleratios, or because the target' +
	'axis has fixed range.');

I can't imagine a scenario where constrain:'domain' would lead to an infinite loop.

@alexcjohnson @plotly/plotly_js can you think any?

[alexcjohnson]

The prohibition on fixedrange wasn't due to infinite loops, it was just that if you've fixed a range and constrained another range based on that, the second range would be fixed too, at least in span.

I suppose if both axes are fixedrange and constrain: 'domain' (as in your mock) there are no concerns: one of them will get a reduced domain but both will keep the specified ranges. If one is fixedrange (and constrain: 'domain'), the other one could in principle be either constrain, and it would need to be restricted to allow pan but not zoom.

[etpinard]

Will be part of #3506