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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)