OB-I diagram, with our empirical separation line. On all plots, the black curve and the grey shaded region represent our empirical separation line (see Eq. (2)). Left: FADO-SDSS sample (orange dots), as described in Sect. 2.1. The error bar represents the median error for all the points. Each contour represents 25% more of the sample, from the smallest to the largest. Right: NII-SF galaxies (top) and NII-AGN galaxies (bottom) that we used to define the empirical line.