Line-ratio diagnostic diagram of R23 = ([O III]λ5007+[O II]λ3727)/Hβ versus O32 = [O III]λ5007/[O II]λ3727). Obscured and unobscured AGNs are plotted with the same symbols as in the previous figures. Purple open circles and dark green triangles represent galaxies with high (12+log(O/H) > 8.8) and low (12+log(O/H) < 8.8) metallicity taken from Kobulnicky & Kewley (2004). Golden open squares represent the corresponding values for extreme emission-line galaxies out to z ∼ 1 in the zCOSMOS survey (Amorín et al. 2015). Light green filled circles represent high-redshift galaxies (4 <  z <  6) with 12+log(O/H) < 8.5 from the JWST/NIRSpec survey (Nakajima et al. 2023). Grey contours show the corresponding distribution for MEx-selected AGNs without X-ray emission with the same redshift and stellar-mass limits as our AGN sample. The strong line diagnostic curve indicates the colour-coded metallicity values in solar units (Z/Z_⊙) taken from Curti et al. (2017).