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)