Intensity ratios trends predicted by CLOUDY as a function of the logarithm of the metallicity, for fixed physical parameters U and n_H. Left panel: ratios of low/intermediate ionization potential BC lines. Right panel: ratios for high ionization potential emissions (BLUE components). In both panels, the stars represent the ratios predicted by CLOUDY. The dashed lines are the interpolation between the CLOUDY values obtained with a cubic spline, the thick solid line segments in each trend line show the range of line ratios covered by our sample, according to the measured flux ratios.