Mass–metallicity relation for the MaNGA star-forming galaxies considered in this work with (right) and without (left) using the [S III] lines via an ionisation parameter prior as discussed in the text. Metallicities are computed for each galaxy at 0.8 R_e. The colour-coding indicates the ionisation parameter (at 0.8 R_e). A histogram of the metallicity distributions for all galaxies is shown in grey on the right. In the case where the [S III] lines are not used (left panel) there is a striking correlation at fixed mass between metallicity and ionisation paramater. To visually highlight this correlation the colour-coding has been smoothed according to the LOESS recipe of Cappellari et al. (2013). The solid black lines show the median metallicity in bins of M_⋆, while the dashed magenta lines are the best-fit using the parametrisation proposed in Curti et al. (2020b).