vs. O/H. Left panel: thin shell models (fr = 3.0), right panel: filled sphere models (fr = 0.03). The solid red dot-dashed line corresponds to the relation from Dopita & Evans (1986), the green dashed line to Dopita et al. (2006), and the red dotted line to a fit to Fig. 3 from Pérez-Montero (2014). The color code follows Q_{0 / 1}. Our fit is shown with the blue line; it is obtained considering only the Q_{0 / 1} > 0.55 regions (see text for the corresponding values).