[Fe/H] vs. R_Gal distribution close to the Galactic plane (| Z_Gal | < 0.3 kpc) for five bins in age (from left to right, as indicated in each panel). Top row: data compilation. The CoRoGEE sample is shown in blue: for each star, we have calculated the fraction of the age PDF enclosed in each age bin – this fraction corresponds to the size of each dot and its transparency value. For comparison, we also show the open cluster compilations of Genovali et al. (2014) and Magrini & Randich (2015) as green pentagons, the subgiant sample from Bergemann et al. (2014, red symbols), and Galactic Cepheids (Genovali et al. 2014; black symbols). The solar-neighbourhood FGK dwarf sample of Bensby et al. (2014) is plotted as orange symbols: the large orange triangles and their error bars denote the median metallicities and the 68% quantiles, while small triangles represent stars that fall outside this range. Second row: mock CoRoGEE sample from the chemodynamical simulation of Minchev, Chiappini, & Martig (2013, 2014a, MCM), including typical observational errors in age, distance, and metallicity (Anders et al. 2017, 2016). Third row: full MCM simulation without errors, and the underlying chemical-evolution model of Chiappini (2009).