log([Oiii]λ5007/Hβ) vs. log([Nii]λ6583/Hα) diagnostic diagram (BPT, Baldwin et al. 1981). a) Low-metallicity candidates with 12 + log O/H ≤ 7.65. Magenta filled circles denote the galaxies with oxygen abundances derived using the direct method, and magenta open circles indicate galaxies with oxygen abundances derived with the semi-empirical method by Izotov & Thuan (2007). Error bars at the lower left corner represent the mean range of errors for individual galaxies. b) XMD candidates with 12 +log O/H ≤ 7.35. XMD candidates with metallicities derived by the direct and semi-empirical methods are denoted by red filled circles and red open circles, respectively. The galaxies with the [O iii]λ4363 Å emission lines detected at a level higher than 3σ are denoted by large red filled symbols and galaxies detected at 2–3σ levels by small red filled circles. We also show the XMD emission-line galaxies and candidates with 12 +log O/H ≤ 7.35 by Izotov & Thuan (2007), Izotov et al. (2012) and Guseva et al. (2015) by small blue circles. The well-known metal-deficient BCDs I Zw 18, SBS 0335–052W, and SBS 0335–052E are represented by filled blue triangles and are labelled. The extremely metal-poor dwarf galaxies Leo P (Skillman et al. 2013) and AGC 198691 (Hirschauer et al. 2016) are plotted by a large open black square and a large open black diamond, respectively. For comparison we also plot in a) and b) a sample of 803 luminous compact galaxies (LCGs, black dots, Izotov et al. 2011a). The dashed line from Kauffmann et al. (2003) separates star-forming galaxies (SF) from active galactic nuclei (AGN).