(a) Evolution of emission-line intensities, as labelled, normalized to the Hβ intensity for Z= 0.0001 (SAL2) models as a function of the logarithm of the ionization parameter. Solid lines correspond to models with n_H= 10 cm⁻³ and dotted lines correspond to models with n_H= 100 cm⁻³. (b) Same as Fig. 8(a) but for Z= 0.0004. (c) Same as Fig. 8(a) but for Z= 0.004. (d) Same as Fig. 8(a) but for Z= 0.008. (e) Same as Fig. 8(a) but for Z= 0.02.