Same as Fig. 3, showing the mean log(O/Ar) values of the high- (blue) and low- (red) extinction PNe but now also showing the linear fit with uncertainty shaded for the low-extinction sample. For the high-extinction PNe, the dashed blue line shows a third-order polynomial curve that better fits the lower mean log(O/Ar) values of the high-extinction PNe in the 12 + log(Ar/H) = 6.1–6.3 range. The [Ar/H] abundances and [O/Ar] ratios with respect to the solar values are marked on the top and right axes, respectively.