Distribution of the photoionization models computed in Feltre et al. (2016) (circles) and Nakajima & Maiolino (2022) (stars) with respect to the line ratios reported in Fig. 1 (top panel), Fig. 2 (central panel), and Fig. 3 (bottom panel). The photoionization models for SFG are reported on the right, while the AGN models are on the left. The points are colour-coded according to their metallicity (the stronger the colour, the higher the metallicity), and the marker size depends on the ionization parameter (the larger the marker, the higher the ionization parameter). The maximum and minimum values of Z and log U for the two classes of models are reported in the top part of the top panels.