Support line breaks in axis labels

[miwig]

Axis labels are drawn using ctx.fillText, which apparently does not support line breaks. It would be nice if uPlot could work around this automatically.

[leeoniya]

should be pretty easy to add. just copy/paste a few bits from the axis renderer:

uPlot/src/uPlot.js

Lines 1804 to 1818 in 852829e

	let _parts = val.indexOf("\n") == -1 ? [val] : val.split(/\n/gm);
	for (let j = 0; j < _parts.length; j++) {
	let text = _parts[j];
	if (angle) {
	ctx.save();
	ctx.translate(x, y + j * lineHeight); // can this be replaced with position math?
	ctx.rotate(angle); // can this be done once?
	ctx.fillText(text, 0, 0);
	ctx.restore();
	}
	else
	ctx.fillText(text, x, y + j * lineHeight);
	}

want to make a PR?