代码片段： Stem.prototype.draw = function(ctx) {   var len, ctrlPoint, point;   len = this.points.length - 1;   ctrlPoint = {x: 0, y: 0};      ctx.save();   ctx.strokeStyle = this.color;   ctx.beginPath();   ctx.moveTo(this.points[this.points.length - 1].x, this.points[this.points.length