Same as Fig. 7 but showing the extinction (top), oxygen abundance (centre) and density (bottom panel) of the H II regions. Central panel: a linear fit to the data (in black) with a 95% confidence interval (shaded grey area). We also indicate the best fit parameters in the upper right corner. Bottom panel: the grey dashed line in the bottom panel indicates the sensitivity limit of the density determination method: points below this lines are consistent – within measurement uncertainty – with n_e ≃ 1 cm⁻³. Error bars span the first to third quartile of a distribution of n_e obtained from 1000 Monte Carlo realisations of the [S II] ratio (see Sect. 5).