Mean log(O/Ar) values of the low-extinction PNe (red) over the 2–30 kpc radial range, the high-extinction PNe (blue) within R_GC < 14 kpc, and the new two-infall chemical evolution model for the M31 disc (see Sect. 4.2 for details), coloured by lookback time: the fiducial model on the left and the alternative model with a weaker first star formation on the right. The [Ar/H] abundances with respect to the solar values are marked on the top and right axes, respectively. More details on the chemical evolution models are provided in Appendix B.