function orient(point, origin) {
	var x0 = origin.x;
	var y0 = origin.y;

	if (x0 === null) {
		return {x: 0, y: -1};
	}
	if (y0 === null) {
		return {x: 1, y: 0};
	}

	var dx = point.x - x0;
	var dy = point.y - y0;
	var ln = Math.sqrt(dx * dx + dy * dy);

	return {
		x: ln ? dx / ln : 0,
		y: ln ? dy / ln : -1
	};
}
https://github.com/chartjs/chartjs-plugin-datalabels/issues/70