So-called fundamental plane by Hunt et al. (2016), as derived by PCA of a sample of ∼1000 low metallicity starbursts, shown as a green line. The distribution of SDSS galaxies from M10 is overplotted with their median metallicity values in mass bins shown in red. The high-z samples with a significant number of galaxies and z <  3 and measured N2 index used by Hunt et al. (2016) are also reported; the zCOSMOS galaxies at z ∼ 0.25 from Cresci et al. (2012) are indicated in orange and the KBSS z ∼ 2.3 galaxies from Steidel et al. (2014) in blue. All metallicities have been derived with the same calibration based on N2 (Pettini & Pagel 2004). Small dots represent the single galaxy measurements, while the larger circles the medians in mass bins. The general population of local star-forming galaxies, as traced by SDSS, as well as the higher z samples are not well reproduced by the plane.