Abundance ratios as a function of the stellar metallicity colour coded according to the stellar effective temperature. The panels show the X1/X2 elemental ratios calculated before (left) and after (right) correcting the abundances from the trends with T_eff (see discussion in the text). The black dashed lines indicate the solar values and the magenta solid line represents the lower limit of C/O = 0.8 for carbon-rich stars.