The M31 PN distribution in the log(Ar/O) versus 12 + log(Ar/H) plane. Left: oxygen-to-argon abundance ratio, log(Ar/O), plotted against argon abundance for the high- (blue) and low- (red) extinction PNe in M31. Right: high- (blue) and low- (red) extinction PNe binned separately as a function of their 12 + log(Ar/H) values. For each bin the sigma-clipped mean log(O/Ar) is plotted along with its error (error bar along the y axis; see Sect. 3.1). The error is computed as the error on the mean log(O/Ar) value added in quadrature to the mean observation error of the PNe that remain in each bin, after the sigma-clipping. Clipped PNe with log(O/Ar) values that are more than 2 sigma beyond the mean values are marked with a lighter shade. Error bars along the x axis capture the rms of PNe in each argon abundance bin. Red and blue error bars parallel to the x axis indicate the width of the bins in 12 + log(Ar/H) for the low-extinction (red) and high-extinction (blue) PN samples.