Plot the history of the ion-cell relaxation.
        kwargs are passed to the plot method of :class:`HistFile`

        Return `matplotlib` figure or None if hist file is not found.

def plot_ioncell_relaxation(self, **kwargs):
        """
        Plot the history of the ion-cell relaxation.
        kwargs are passed to the plot method of :class:`HistFile`

        Return `matplotlib` figure or None if hist file is not found.
        """
        with self.ioncell_task.open_hist() as hist:
            return hist.plot(**kwargs) if hist else None