Upper row: same as Fig. 5b but the points are colour-coded according to the HI fraction; blue and red points come from HI and H₂ dominated regions, respectively. The VSF law appears tight and straight even if only HI-dominated regions are considered. Lower row: VSF law between total gas and SFR. The solid black line is the best-fit relation with slope α and orthogonal intrinsic scatter σ_⊥ (dashed lines). The grey bands show 1σ and 2σ uncertainties on the fit. In the left panel, ρ_SFR is calculated with the constant h_SFR, while in the right panel h_SFR(R) flares with radius (Eq. (17)). The volume densities radial profiles are shown as contours containing 95% (yellow), 75% (orange), 50% (red), and 25% (dark red) of the data points.