Evolution of the stellar metallicity gradients. Upper panel: median slopes of the metallicity profiles for all simulated discs (black circles) and for those in the low-mass (red squares) and high-mass (blue diamonds) subsamples as a function of redshift. For comparison we include the observational estimations reported by Maciel et al. (2003) for the Milky Way (green open circles) and by Magrini et al. (2016; orange stars) for nearby galaxies. Estimations of the median metallicity gradients (open triangles) performed using the ages and spatial distributions of stars at z ~ 0 are included (see text for details). Lower panel: similar estimations for the normalised metallicity gradients. In both panels, the error bars are estimated using a bootstrap technique. For the sake of clarity, the simulation points are slightly offset horizontally in both panels.