[Mg/Fe] versus [Fe/H] (top panel) and [Fe/Mg] versus [Mg/H] (bottom panel) in the solar neighbourhood. Theoretical predictions are from Models 1 [short-dashed (red) lines], 4 [upper solid (blue) line in panel a) and lower solid (blue) line in panel b], and 5 [lower solid (black) line in panel a) and upper solid (black) line in panel b]. Data are from Andrievsky et al. (2010, stars), Lai et al. (2008, open pentagons), Gratton et al. (2003, upside-down triangles), Gehren et al. (2006, open triangles), Reddy et al. (2006, filled squares), Reddy et al. (2003, open squares), and Bensby et al. (2005, filled circles). See the electronic edition of the journal for a colour version of this figure.