Panel a: magnesium-to-oxygen abundance ratio relative to solar value [Mg/O] = log(Mg/O) – log(Mg/O)_⊙ vs. metallicity expressed in 12 + log(O/H). Galaxies with metallicities derived using the direct T_e method are in red; those obtained by the semi-empirical method are in black (see Sect. 3). The average values of [Mg/O] from the direct and semi-empirical methods are shown by solid red and dashed black lines, respectively, and are very similar (–0.279 and –0.256). Panel b: oxygen-to-neon ratio relative to the solar value [O/Ne] vs. 12 + log(Ne/H). Panels c and d: [Mg/Ne] vs. 12 + log(O/H) (panel c) or 12 + log(Ne/H) (panel d). Linear regressions are shown in panels b and d by solid blue and black lines, respectively.