Two separate two-dimensional histograms illustrating dependencies between the amplitudes of different spectral lines. The dependence between the amplitudes of Hα and He I D₃ is shown in the left panel, whereas the right panel depicts the dependence between the amplitudes of Hα and Ca II IR. All amplitudes are normalised to the corresponding local continuum at disk centre. Redder colors indicate a larger density of points, while bluer colors indicate a smaller density. A linear relationship exists in both plots for Hα normalised amplitudes below 0.03. The arrow indicates the interval considered as optically thin for the analysis (see Sect. 3.1).