Temporal evolution of the standard deviation of D, σ_D, for several co-forming groups of stars traced along snapshots (solid lines) and for all stars together (dashed line). Crosses correspond to the gas of the snapshot 0.5 Gyr before the formation of each group and are linked to the curves with dotted lines to guide the eye. Shaded areas show the dispersion for each formation time due to short-term fluctuations, computed as the mean standard deviation of σ_D from intervals with higher-cadence. The innermost 4 kpc and radii beyond 8 kpc are excluded.