Panels a) to d): trends in data we used for the degeneracy line method. In all panels the open circles show the FE12 and filled circles show the FE14 sample. Panels a) and b) show how the median difference in equivalent width for the iron lines ⟨ΔEW_FeI⟩ depends on effective temperature and metallicity. Panels c) and d) show how the slope[(ΔEW_FeI) vs. χ_exc], depends on the same parameters. Contrary to expectations, the slope parameter only depends on metallicity and not on effective temperature (as already noted in D12).