Predicted [Mg/Fe] versus [Fe/H] density maps. Left panel: chemical evolution model for the Milky Way. Right panel: chemical evolution model for UFD galaxy Boötes I, taken as a prototype of the satellites accreted by the MW at early times. The highest density region of the GCE model and the average of UDF model are offset by +0.4 dex (dotted red line and dotted blue line, respectively) to allow a better comparison with the observed abundances. The pink solid line is the LOWESS regression. The circles represent the Precision I sample with Mg under 3D NLTE.