Column densities of CO (top), CH (middle), and CN (bottom) versus column densities of H₂, in the los models. Circles correspond to lines of sight parallel to Y and squares to lines of sight parallel to X. Their colours reflect the mean gas density ⟨n_H⟩ on these lines of sight. Plus signs on the top (CO) plot stand for observational data points (Sheffer et al. 2008; Crenny & Federman 2004; Pan et al. 2005; Lacour et al. 2005; Rachford et al. 2002, 2009; Snow et al. 2008). The dashed lines are power-law fits from Sheffer et al. (2008). The lines of sight parallel to X marked 0, 1, 2, and 3 on the top panel are the same as in Figs. 7 and 8.