Top panels, left-hand side: sequence of model spectra varying only the parameter m_top from −4.0 to − 3.5 dex, the models are: #136 (black), #137 (blue), #138 (yellow), and #139 (red). Top panels, right-hand side: sequence for the parameter m_mid varying from −2.7 to −2.0 dex, models: #121 (yellow), #126 (blue), #138 (red), and #153 (black). Bottom panels: corresponding values of I_line against m_top for Na I D (blue crosses), Hα (red pluses), and Ca II IRT (green diamonds) for the top panels. Additionally the linear fits are plotted: the Na I D fit is plotted by the blue dashed line, the Hα fit by the red dash-dotted line, and the Ca II IRT fit by the green dotted line.