Longitude–velocity maps of the simulation observed from the Sun’s position (placed at the origin of the overlayed coordinate system in Fig. 9). The bar major axis is rotated by 28 degrees from the LOS passing through the Galactic centre. Overlayed blue dashed lines are the spiral arm tracks of Reid et al. (2016, 2019) and McClure-Griffiths et al. (2004). The green points are the terminal velocities from H I and CO observations. Top: full diagram. Bottom: zoom-in between l = −60° & +60°. The left plot only takes into account gas that lies at R < 6 kpc from the centre, and the right one only considers gas at R > 6 kpc.