Display parcoords deselected lines

[archmoj]

This PR could fix the issue #3099 i.e. before the implementation of hovermode for parcoords.

@alexcjohnson

[alexcjohnson]

Looks great @archmoj ! Can you add a test to 🔒 this fix? I was going to suggest modeling it after this test:

plotly.js/test/jasmine/tests/parcoords_test.js

Lines 655 to 663 in 3f95f04

	d3.selectAll('.parcoords-lines').each(function(d) {
	var imageArray = d.lineLayer.readPixels(0, 0, d.model.canvasWidth, d.model.canvasHeight);
	var foundPixel = false;
	var i = 0;
	do {
	foundPixel = foundPixel || imageArray[i++] !== 0;
	} while(!foundPixel && i < imageArray.length);
	expect(foundPixel).toEqual(false);
	});

(with the inverse expectation of course, foundPixel should be true) And while I think the inner part of that test may still be good, the test itself is broken - it doesn't bother to test that d3.selectAll('.parcoords-lines') actually yields any elements! (add the line expect(d3.selectAll('.parcoords-lines').size()).not.toBe(0); and it fails) So the .each never runs. Must have been refactored so the .parcoords-lines class is never used.

Don't spend any time fixing that test now unless it's trivial, but please do come up with a similar test showing that all the layers have some data in them after an edit. No need to simulate clicking the modebar button, some other edit like the one I mentioned in #3099 Plotly.relayout(gd,'paper_bgcolor','#eff') will suffice.

[alexcjohnson]

@archmoj before your patch this fails with:

Expected false to be true, 'after relayout - gl-canvas gl-canvas-context'.

[alexcjohnson]

previously this test didn't bother checking that it was actually doing anything! d3.selectAll('.parcoords-lines') was an empty set - this must have been refactored at some point - so the .each block never ran. Updated it to not only ensure that it's doing something (expect(canvases.size()).toBe(3, msg); up above) but also to test that something changed (_assertVisibleData(true, 'initial') before the change, _assertVisibleData(false, 'no panels') afterward).

[archmoj]

Thank you @alexcjohnson
💃