Galactocentric radial distribution of log(O/Ar) values for HII regions (top) and for high- and low-extinction PNe (middle and bottom, respectively) in the M31 disc from the PN catalogue published in Paper IV. The best-fitting radial log(O/Ar) gradient is shown for HII regions (black) and high- (blue) and low-extinction (red) PNe. Middle panel: displays three independent linear fits for three radial ranges: the solid line is the linear fit to the entire dataset; for within 14 kpc and beyond 18 kpc, the linear fits are shown with dashed lines.