Same as Fig. 3, showing the mean log(O/Ar) values of the high- (blue) and low- (red) extinction PNe, but now showing only high-extinction PNe in the inner disc (R_GC < 14 kpc; left), in the starburst region (14 ≤ R_GC ≤ 18 kpc; middle), and in the outer disc (R_GC > 18 kpc; right). The blue error bars parallel to the x axis indicate the width of the bins in 12 + log(A/H) for the high-extinction (blue) PN sub-samples in the different annuli.