Carbon-to-iron (left panel) and carbon-to-magnesium (right panel) abundance ratios as functions of [Fe/H] and [Mg/H], respectively. The red and blue symbols represent abundance estimates from targeted observations of thin- and thick-disc stars, respectively (triangles correspond to Nissen & Schuster 2010; Nissen et al. 2014; the squares correspond to Bensby & Feltzing 2006, for C, and Bensby et al. 2014, for Mg; the circles correspond to Amarsi et al. 2019). The red and blue lines represent trends of abundance ratios of thin- and thick-disc stars, respectively, which were derived from: (i) nearly 1300 thin-disc and 100 thick-disc dwarf stars from GES iDR5, which were selected on the basis of chemical criteria (continuous lines; Franchini et al. 2020); (ii) more than 12 000 stars from GALAH DR2, divided into high-Ia and low-Ia sequences (dashed lines; Griffith et al. 2019). No zero-point offsets were applied to the data.