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 – 1].y);
  for (var i = len; i >= 1; i–) {
    point = this.points[i];
    ctrlPoint.x = (point.x + this.points[i – 1].x) / 2;
    ctrlPoint.y = (point.y + this.points[i – 1].y) / 2;
    ctx.quadraticCurveTo(point.x, point.y, ctrlPoint.x, ctrlPoint.y);
    ctx.lineWidth = i * 1.1;
    ctx.stroke();
    ctx.fillStyle = \’red\’;
  }
  ctx.restore();
}