Left panels: carbon-to-oxygen (upper panel) and nitrogen-to-oxygen (lower panel) ratios as functions of [O/H] in the solar neighbourhood. Right panels: [C/Fe] (upper panel) and [N/Fe] (lower panel) as functions of [Fe/H] in the solar neighbourhood. Data are from Spite et al. (2005, empty stars), Lai et al. (2008, empty pentagons), Fabbian et al. (2009, crosses), Akerman et al. (2004, filled tringles), Israelian et al. (2004, open triangles), Reddy et al. (2006, filled squares), Reddy et al. (2003, open squares) and Bensby & Feltzing (2006, filled circles). Model predictions are shown for Models 1 [short-dashed (red) lines], 3 [long-dashed (black) lines], 4 [solid (blue) lines], 5 [solid (black) lines], 6 [dot-dashed (black) lines], 9 [dotted (black) lines], 10 [short-dashed-long-dashed (blue) lines], and 15 (thick solid lines). See the electronic edition of the journal for a colour version of this figure.