test plotting: assert each axes has 2 lines

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https://api.github.com/giocaizzi/py3dinterpolations/blob/4544c573a8e987c599122c5a0906450b7e2bfa18/tests/plotting/test_plotting.py#L43

    ).preprocess()

    # patch the reverse_preprocessing function
    # returning  a GriddData object
    # created with a sample of the test_data
    with patch(
        "py3dinterpolations.plotting.plotting.reverse_preprocessing",
        return_value=GridData(test_data.sample(n=50, random_state=42)),
    ):
        fig = plot_downsampling(gd, preprocessed_gd)

    # assert is figure
    assert isinstance(fig, Figure)

    # assert figure has n visible axes
    n = len(gd.data.index.get_level_values("ID").unique())
    visible_axes = [ax for ax in fig.axes if ax.get_visible()]
    assert len(visible_axes) == n

    # # TODO assert each axes has 2 lines