Difference in non-LTE Mg abundance Δ[Mg/Fe]_NLTE versus stellar metallicity for a fixed LTE difference Δ[Mg/Fe]_LTE (=0, 0.2 and 0.4 in the first, second and third row respectively), between two stars with T_eff/log g parameters shown at the bottom right of each plot. The LTE difference Δ[Mg/Fe]_LTE is represented by the dotted line. Colours indicate the LTE abundance [Mg/Fe]_LTE for each star shown at the top-left of each plot. Four different Mg i lines were used: 5711(Λ), 7691(◯), 5528 (△) and 8806 (▽) Å. Examples of individual stars at various evolutionary stages were selected: main sequence (MS), turn off point (TOP), red giant branch (RGB) and base of the RGB (bRGB).