Panel a) and b) metallicity as function of orbital and guiding centre radius, respectively. The probability of a star to belong to the thin disc is represented by colour. Stars with halo membership higher than 50% are plotted in black. Filled diamonds and squares identify stars having V_LSR <  −40   km   s^-1 and >20   km   s^-1, respectively (cf. with Fig. 20). Panel c) cumulative metallicity gradient ([Fe/H]  and  [M/H] ) when including stars of increasingly older ages. Error bars are shown in one representative case. The gradient is computed using the mean orbital radius of stars as baseline. A kinematic cut to exclude halo stars (as described in the text) is also adopted for comparison. Panel d) same panel c), but using the guiding centre radius as baseline. Panel e) metallicity gradient centred at different ages, weighting all other stars with a Gaussian of width 1.5 Gyr and using the mean orbital radius as baseline. Panel f) same as panel e), but using the guiding centre radius. Only stars with well determined ages (Sect. 3) are used in all instances.