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)
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.