Joint histograms of the molecular column densities, abundances, or their ratios as a function of the visual extinction for ¹²CO, ¹³CO, C¹⁸O, HCO⁺, and H¹³CO⁺, from top to bottom. The three first columns show the column densities for the inner, outer (foreground or background), and total (inner plus twice outer) column densities, while the last column shows the mean abundance, namely, the total column density divided by the H₂ column density deduced from the visual extinction. The dashed vertical lines mark the visual extinction at 3, 7, and 16 mag. The red plain lines show the species basic abundance. The black lines show the piecewise power-law fits of the column densities within the A_V ranges specified above (see Table 6) and the ±3σ intervals of the fits. These fits can be directly converted to an abundance vs visual extinction relation.