Present-day residual azimuthal variations in iron computed at different galactocentric distances for the multiple spiral structures of Models A, A+C1, A+C1, and A+C3, compared with the metallicity variation found by Poggio et al. (2022) when analysing GSP-Spec abundances of Gaia DR3 (Gaia Collaboration 2023). Horizontal lines with the same colour indicate the 10th and 90th percentiles of the metallicity variation as computed by Poggio et al. (2022) at different galactocentric distances. In each panel, the shaded grey area indicates the region spanned by Poggio et al. (2022) data. In the zoom-in plot associated with the fourth panel, we report the Poggio et al. (2022) data, indicating the median metallicity for Sample A as a function of Galactic azimuth for different galactocentric distances (after the median metallicity of the stars for each ring has been subtracted).