'make a twinx axes of self. This is used for twinx and twiny.'

def _make_twin_axes(self, *kl, **kwargs):

    from matplotlib.projections import process_projection_requirements
    kl = ((self.get_subplotspec(),) + kl)
    (projection_class, kwargs, key) = process_projection_requirements(self.figure, *kl, **kwargs)
    ax2 = subplot_class_factory(projection_class)(self.figure, *kl, **kwargs)
    self.figure.add_subplot(ax2)
    return ax2
