docs: def plot_iso(axis, step, var):
      """Plot isocontours of scalar field.
  
      Args:
          axis (:class:`matplotlib.axes.Axes`): the axis handler of an
              existing matplotlib figure where the isocontours should
              be plotted.
          step (:class:`~stagpy.stagyydata._Step`): a step of a StagyyData
              instance.
          var (str): the scalar field name.
      """
      xmesh, ymesh, fld = get_meshes_fld(step, var)
      if conf.field.shift:
          fld = np.roll(fld, conf.field.shift, axis=0)
      axis.contour(xmesh, ymesh, fld, linewidths=1)