code: function(ratio) {
  			//since we used _addTween() inside init function, it created some property tweens that we'll update by calling the parent prototype's setRatio() (otherwise, the property tweens wouldn't get their values updated). this._super refers to the TweenPlugin prototype from which the plugin inherits (not that you need to worry about that).
  			this._super.setRatio.call(this, ratio);
  
  			//now manually set the alpha
  			this._target.alpha = this._alphaStart + this._alphaChange * ratio;
  		}
docs: [optional] - called each time the values should be updated, and the ratio gets passed as the only parameter (typically it's a value between 0 and 1, but it can exceed those when using an ease like Elastic.easeOut or Back.easeOut, etc.). If you're using this._super._addTween() for all your tweens and you don't need to do anything special on each frame besides updating those values, you can omit this "set" function altogether.