Abundance ratios of the refractory elements Mg, Si, and Fe over the volatile element Zn (left panels) and S (right panels), plotted versus the absolute abundance of the volatile element. Filled symbols with error bars: measurements in DLA systems obtained from the (Zn+S) sample (black circles), the  sample (red diamonds), and the  sample (blue squares); see Sect. 3.3. Curves: predictions obtained from the models of galactic chemical evolution selected by our procedure (same legenda as in Fig. 2, top panel).