Returns the histogram axes, creating it only on demand.

def hax(self):
        """
        Returns the histogram axes, creating it only on demand.
        """
        if make_axes_locatable is None:
            raise YellowbrickValueError((
                "residuals histogram requires matplotlib 2.0.2 or greater "
                "please upgrade matplotlib or set hist=False on the visualizer"
            ))

        divider = make_axes_locatable(self.ax)

        hax = divider.append_axes("right", size=1, pad=0.1, sharey=self.ax)
        hax.yaxis.tick_right()
        hax.grid(False, axis='x')

        return hax