Well, the thing is you can have two y axes, but they must be simple transformations one another. The reason being (iirc, don't want to put words into Hadley Wic

Well, the thing is you can have two y axes, but they must be simple transformations one another. The reason being (iirc, don't want to put words into Hadley Wickham's mouth) is that if you cannot provide a transformation from one y axis to the other, then the data shouldn't be in one plot to begin with (as it then shows clearly distinct data).