Evolution of the oxygen gradient for several galaxies, derived from the PN and HII region gradients, adapted from Stanghellini et al. (2010). The data points for M 81 have been updated considering the gradient derived here, and include all slopes of Fig. 5: squares are for best statistical gradients, with converging fits and χ² /(degrees of freedom) ~1; the cross at t = 0 indicates the location of the gradient from the middle panel of Fig. 5, where only low-uncertainty data points have been included. The data points for NGC 300 are from Stasinska et al. (2013).