docs: function getDistanceMetricForAxis(axis) {
  	var useX = axis.indexOf('x') !== -1;
  	var useY = axis.indexOf('y') !== -1;
  
  	return function(pt1, pt2) {
  		var deltaX = useX ? Math.abs(pt1.x - pt2.x) : 0;
  		var deltaY = useY ? Math.abs(pt1.y - pt2.y) : 0;
  		return Math.sqrt(Math.pow(deltaX, 2) + Math.pow(deltaY, 2));
  	};
  }