Galactocentric velocity versus longitude. Left panels: individual stars of the Mg- rich/poor sequences (as defined in the main text) are displayed with red/blue color in the upper/lower panel. In each case, the mean linear trend of the data is displayed as a solid colored line (estimated with the Theil–Sen estimator for robustness), and the 2σ dispersion band as a shaded area. For visual aid, in each panel the linear trend of the other distribution is displayed as a dashed black line. Right upper panel: whole close-to-the-plane sample (thus the union of all stars displayed in the left panels) is displayed as green points with its linear trend as a solid line of the same color. Black crosses depict the subsample of low-Mg stars (in the metallicity range where Mg-bimodality is seen; see main text), with its linear trend displayed as a dashed black line. Right lower panel: sample of disk stars, selected as 3.5 <  R_GC <  5.0 kpc and |Z| < 0.5 kpc, displayed as orange points, with its linear fit as a solid orange line. Black crosses and the dashed black line have the same meaning as in the upper panel.