Barred-disk model. Distribution of the azimuthal velocity and metallicity after an evolution of 0, 0.1, 0.4 Gyr. Left panels: colour contour plot showing the rotation-metallicity distribution, V_φ vs. [Fe/H], of the particles in the solar annulus, 8 kpc  < R < 10 kpc and 1.5 kpc  <  | z |  < 2.0 kpc. The dashed white line indicates the linear fit in the central region of the distribution, including all particles with  −0.55 <  [Fe/H]  <  − 0.35 and V_φ > 50 km s^-1. Right panels: the colour contour plot represents the distribution of the mean original radius,  ⟨ R₀ ⟩ , as a function of ([Fe/H],V_φ) of the same particles shown in the left plot. Here, the dotted white lines mark the iso-contours at  ⟨ R₀ ⟩  = 6 kpc, 8 kpc, 10 kpc, and 12 kpc. The black contours indicate the iso-density levels of the ([Fe/H],V_φ) distribution shown in the left panel.