SFR versus M_⋆ for the BPT-SF (upper-panel) and BPT-non-SF (lower-panel) samples identified on the basis of [N II]-BPT. The data points are colour coded with respect to the gas-phase metallicity (12 + log(O/H)) using the O3N2 diagnostic described in Sect. 2.2. On both panels, the black MS line is derived from fitting a median to the SF galaxies. The error bars in the bottom-right corner indicate the median uncertainties on SFR and M_⋆ within each subsample and are estimated from the percentiles provided in the MPA-JHU catalogue. The uncertainty on gas-phase metallicity is of the order of 0.09 dex.