code: def plot_ioncell_relaxation(self, **kwargs):
          
          with self.ioncell_task.open_hist() as hist:
              return hist.plot(**kwargs) if hist else None
docs: 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.