Uncaught TypeError: Cannot read property 'LineCap' of undefined

估计是你的节点上没有添加cc.Graphics组件，this.ctx=this.getComponent("cc.Graphics")的时候得到的是undefined，下一行this.ctx.lineCap就会报这个错误，无法读取undefined的属性LineCap。你加这样一行试试：

this.ctx = this.getComponent("cc.Graphics");
if (!this.ctx) {
    this.ctx = this.addComponent("cc.Graphics");
}
this.ctx.lineCap = cc.Graphics.LineCap.BUTT;