Create a twin Axes sharing the xaxis.
Create a new Axes with an invisible x-axis and an independent y-axis positioned opposite to the original one (i.e. at right). The x-axis autoscale setting will be inherited from the original Axes. To ensure that the tick marks of both y-axes align, see
The newly created Axes instance
For those who are 'picking' artists while using twinx, pick events are only called for the artists in the top-most Axes.