docs: def plot_vec(axis, step, var):
      """Plot vector field.
  
      Args:
          axis (:class:`matplotlib.axes.Axes`): the axis handler of an
              existing matplotlib figure where the vector field should
              be plotted.
          step (:class:`~stagpy.stagyydata._Step`): a step of a StagyyData
              instance.
          var (str): the vector field name.
      """
      xmesh, ymesh, vec1, vec2 = get_meshes_vec(step, var)
      dipz = step.geom.nztot // 10
      if conf.field.shift:
          vec1 = np.roll(vec1, conf.field.shift, axis=0)
          vec2 = np.roll(vec2, conf.field.shift, axis=0)
      if step.geom.spherical or conf.plot.ratio is None:
          dipx = dipz
      else:
          dipx = step.geom.nytot if step.geom.twod_yz else step.geom.nxtot
          dipx = int(dipx // 10 * conf.plot.ratio) + 1
      axis.quiver(xmesh[::dipx, ::dipz], ymesh[::dipx, ::dipz],
                  vec1[::dipx, ::dipz], vec2[::dipx, ::dipz],
                  linewidths=1)