joint.all.js
function(len) {
	    var s = (len || 1) / this.magnitude();
	    this.x = s * this.x;
	    this.y = s * this.y;
	    return this;
        }
function(len) {
	    var s = (len || 1) / this.magnitude();
	    this.x = s * this.x;
	    this.y = s * this.y;
	    return this;
        }
Scale the line segment between (0,0) and me to have a length of len.