The dependence of the mass–metallicity relation on the galaxy physical parameters, including SFR, |$D_{\rm n}(4000)$|⁠, E(B − V) and gas mass fraction. In each mass bin, we divide galaxies into two groups by the parameter. We calculate the median of mass and metallicity for each group in each bin. The median absolute deviation from median value is used to estimate the spread and represented as error bar. The deviation shown in the head of each panel represents the average offset in each bin.