Variation of the integrated line profiles as a function of the column density, N. Each panel corresponds to one particular line or multiplet, i.e. H I Lyαλ1216, Mg IIλλ2796, 2803, Si IIλλ1190, 1193, Si IIλ1260, Fe IIλλ2586,2600, and C IIλ1334. All lines have been normalised to the continuum intensity. Dotted grey lines depict the input emission and vertical lines show the location of the resonances. On the x-axis, frequencies are expressed in terms of velocity offset relative to the resonant wavelength λ₀ (the bluest line is used in case of multiplets), namely $V=c(\frac{\lambda }{{\lambda }_0}-1)$. For Fe II, the inset shows the fluorescent lines associated with Fe IIλλ2586,2600 which are emitted at much longer wavelengths (see Table 1).