[α/Fe]-vs.-[Fe/H] diagram for five different bins in Galactocentric distance R_Gal. Top row: the CoRoGEE sample. The colours represent our stellar age estimates: blue indicates stars younger than 3 Gyr, red stands for stars older than 8 Gyr, and yellow for intermediate ages. The point size and transparency of each data point encode the age uncertainty, i.e., a smaller and more transparent symbol corresponds to a smaller probability of belonging to the particular age bin. Second row: the sophisticated MCM mock sample. Third row: all mock particles from the MCM N-body simulation, shown as grey dots. The solid lines represent the predictions of the underlying thin disc chemical-evolution model by Chiappini (2009) for stars born in the corresponding Galactocentric distance bin. The colour-code represents the age; the shaded regions along the lines correspond to a 2σ-confidence band, given the typical uncertainties in [Fe/H] and [α/Fe]. The dashed lines show the chemical tracks of Chiappini (2009) for the thick disc. The error bar in the upper right of each panel represents the typical (internal) uncertainty of the chemical abundances. Fourth row: the simple MCM mock sample, without simulated age uncertainties. Last row: the simple MCM mock sample, with age uncertainties.