Add plotlyServerUrl to baseUrl

[chriddyp]

Fixes #2759 (comment)

I haven't tested this.

[etpinard]

Ok. I'll take it from here.

Will be part of v1.39.0.

[etpinard]

These tests aren't great, but I couldn't find an easy way to mock the form submit in

plotly.js/src/plots/plots.js

Lines 214 to 245 in bcab36e

	plots.sendDataToCloud = function(gd) {
	gd.emit('plotly_beforeexport');
	var baseUrl = (window.PLOTLYENV && window.PLOTLYENV.BASE_URL) || 'https://plot.ly';
	var hiddenformDiv = d3.select(gd)
	.append('div')
	.attr('id', 'hiddenform')
	.style('display', 'none');
	var hiddenform = hiddenformDiv
	.append('form')
	.attr({
	action: baseUrl + '/external',
	method: 'post',
	target: '_blank'
	});
	var hiddenformInput = hiddenform
	.append('input')
	.attr({
	type: 'text',
	name: 'data'
	});
	hiddenformInput.node().value = plots.graphJson(gd, false, 'keepdata');
	hiddenform.node().submit();
	hiddenformDiv.remove();
	gd.emit('plotly_afterexport');
	return false;
	};

[etpinard]

thank you @alexcjohnson for the tip!

[alexcjohnson]

good call, that also seems more standard in the built-in API (eg canvas.toDataURL) though we have some other internal routines that use ...Url, like Drawing.setClipUrl.

[alexcjohnson]

Big improvement - thanks @chriddyp, looks great @etpinard 💃