Panel a: diagnostic diagram [O III]λ5007/Hβ–[O II]λ3727/Hβ. Open circles and open triangles are the extremely low-metallicity SFG candidates selected in this paper with oxygen abundances derived by the direct and semi-empirical methods, respectively. Other symbols are the same as in Fig. 1. The shaded region indicates the location of the extremely low-metallicity SFGs with 12 + log O/H in the range 6.62–7.39. Dotted and dashed lines are the lines of constant O₃₂ and of constant R₂₃ − 0.08O₃₂ (or constant 12 + log O/H), respectively, labelled with their values. The values R₂₃ − 0.08O₃₂ = 0.75, 1.5, 2.5, and 3.75 correspond to oxygen abundances 12 + log O/H = 6.69, 6.97, 7.18, and 7.35, respectively. Panel b: diagram O₃₂–R₂₃. Symbols, lines, labels and shaded region follow the definitions provided for panel a.