Illustration of the impact of different calibrations on the observed [α/Fe] vs. [M/H] trends for the samples defined in Sect. 3.2: HotTO (orange), RGB (grey, with the inner and outer thin disc in pink and green, respectively), and Massive (turquoise) samples. Upper panel: Raw data (no calibration). Left middle panel: Calibrated [α/Fe] based on log(g) and no [M/H] calibration. This is the solution used throughout the different sections of this article. Right middle panel: Calibrated [α/Fe] and [M/H] based on log(g). Left lower panel: Calibrated [α/Fe] based on T_eff and no [M/H] calibration. Right left panel: calibrated [α/Fe] and [M/H] based on T_eff.