Distribution of Δ [ Mg i/ M ] = [ Mg i/ M ] − [ Mg i/ M ] _separation in | Z | bins. Δ [ Mg i/ M ] quantifies the distance in [Mg i/M] of a given star to the thin- and thick-disc separating line (see Fig. 2). The black solid zero vertical line marks the position of the separating line. The two vertical dotted lines at Δ [ Mg i/ M ] ± 0.05 dex mark the zone from which stars were excluded for the analysis of radial and vertical gradients. The plot at the right side shows the total number of thin-disc (black dots) and thick-disc (red dots) stars in the same six | Z | bins.