Mass-metallicity relation of 69 830 SDSS galaxies (in the redshift range 0.04 < z < 0.2), shown with a kernel density estimate plot as the shaded region. Markers indicate galaxies in our sample. The dashed red line represents a third degree fit to SDSS data. The dotted blue line indicates the MMR calculated for the MaNGA survey (Barrera-Ballesteros et al. 2017) using the O3N2-M13 indicator (Marino et al. 2013), and recalibrated to the PG16 indicator (see Appendix E).