Density plots of the metallicity for the thick (upper row) and thin (lower row) discs in terms of the formation radius of the stars are shown alongside the evolution of the metallicity gradient with time (solid lines), for galaxy MaGICC-g15784. Stars have been selected according to our spatial cut, as outlined in Sect. 3.1.1. The legend bar above the figure describes the difference in colour for the lines, which are the same in the six panels; each line indicates the behaviour of [Fe/H] with the formation radius for new born stars, at different time steps, as indicated in the colour bar. The density plots from left to right show the results for increasing current radii ranges, between 7 <r< 8 kpc, 10 <r< 11 kpc and 14 <r< 15 kpc. The vertical dashed lines in each panel correspond to these radial bins.