Variation of the gas-to-dust ratio with galactocentric radius, for our fiducial case (panel a), considering a CO/C¹⁸O galactocentric gradient (panel b), and assuming that the abundance of CO follows the radial oxygen gradient, rather than the C/H (panel c). The thick blue lines indicate the best fit, reported in the bottom right corner; the 68% and 95% highest probability density intervals of the fit parameters are indicated by the light and dark yellow-shaded regions, respectively. The intrinsic scatter is indicated by the dashed lines. For comparison with external galaxies log  (O/H) + 12 is shown on the top axis.