[OIII]88μm/[N⁺]122μm line ratio as a function of the metallicity for three gas densities n_H = 10 (circle), 100 (square), and 1000 (triangle) cm⁻³, for the ionization parameters log U varying betwen −4.0 and −1.0 (with increasing size of the symbols), and for the covering factor f_cov = 0 (a), 0.5 (b), and 1 (c). The f_cov = 0 case (a) corresponds to Fig. 5 of Rigopoulou et al. (2018). The shaded region corresponds to the values spanned by this ratio in our samples.