Stellar mass oxygen abundance relation. Emission-line XMD galaxies with 12 + log O/H < 7.35 and our XMD candidates are shown in red and blue. Symbols are the same as in Fig. 3. For comparison, the SDSS DR7 emission-line galaxy sample by Izotov et al. (2014a), where the errors in [O iii]λ4363 emission-line flux do not exceed 25%, are shown in black dots. The linear likelihood regression for this SDSS DR7 sample is plotted with a solid line. The small open squares in magenta are the data derived by Berg et al. (2012). The mass–metallicity relation obtained from 53 000 SDSS star-forming galaxies by Tremonti et al. (2004) is plotted with a dashed line.