Panel a) absolute SDSS g magnitude–oxygen abundance relation for the five galaxies with oxygen abundances derived using the “direct” method (large red filled circles) and for all eight galaxies with oxygen abundances derived with the semi-empirical method by Izotov & Thuan (2007) (large red open circles). We also show the XMD emission-line galaxies with 12 + log O/H ≤ 7.35 by Izotov & Thuan (2007) and Izotov et al. (2012) in small red filled circles. The small open squares in magenta, green, and blue are the data derived by Berg et al. (2012), Pustilnik et al. (2011), and James et al. (2015), respectively (see text). The most-metal deficient BCDs I Zw 18, SBS 0335–052W, and SBS 0335–052E are shown with filled blue triangles and are labelled. One of the most metal-poor dwarf galaxies, Leo P from Skillman et al. (2013) is plotted with a large open black square. For comparison, the “standard” L–Z relation for emission-line galaxies (SDSS and star-forming galaxies from Izotov et al. 2006a; Izotov & Thuan 2007) are shown in black dots with the linear regression to these data (solid line). The relations derived by Richer & McCall (1995) and by Berg et al. (2012) are shown in a dotted line and a dashed line, respectively. Panel b) extinction-corrected Hβ luminosity–oxygen abundance relation. The samples and symbols are the same as in panel a). The solid line represents the linear regression to SDSS galaxies (Izotov et al. 2006a) and to all galaxies from Izotov & Thuan (2007), excluding I Zw 18, SBS 0335–052W, and SBS 0335–052E, and to galaxies from Izotov et al. (2012).