Left panel: log(N/O) vs. [O/H] as a function of both stellar mass and radius in the sample of resolved galaxies of Belfiore et al. (2017; extracted from their Fig. 8). For each mass bin (see the legend for the range in logarithm of the stellar mass of each bin) the uppermost star represents the innermost radial bin (0.0–0.25 R_e while the lower star represents the outermost radial bin (1.75–2.0 R_e). Right panel: log(N/O) vs. [Fe/H] in our Milky Way samples of field stars (blue and pink triangles, thin and thick discs, respectively) and of open clusters (in green clusters with R _GC > 7 kpc and in orange clusters with R_GC ≤ 7 kpc).