Calibration of R₃ = [O III] λ5007/Hβ (panel a), N₂ = [N II] λ6584/Hα (panel b), O₃N₂ = R₃/N₂ (panel c), and N₂S₂Hα = N₂/S₂ ⋅ N₂^0.264 (panel d) diagnostics carried out in this work (solid golden curves). Data points are colour-coded according to the original source catalogues, with golden points associated with PHANGS–MUSE data. The other curves report calibration from the literature for comparison. The dashed extension at low-metallicity shows the extrapolation of our calibrations.