Face-on optical depth in the FUV band versus the total gas mass surface density in units of M_⊙ pc^-2. The colour of each point indicates the oxygen abundance determined from the gradients published by Zaritsky et al. (1994). The black line represents the best fit for the entire sample (Eq. (12)) when taking into account a metallicity-dependent X_CO factor (Eq. (11)). The coloured lines represent the best fit for the entire sample (Eq. (13)) for different values of 12 + log O/H, when fitting the gas column density and the oxygen abundance simultaneously. At the top of each panel the number of datapoints and the Spearman correlation coefficient is indicated.