Residuals (i.e., logarithmic distance) between the observed (and detected) G/D and predicted G/D from the three relations for X_CO,MW (left) and X_CO,Z (right): reference scaling of the G/D with metallicity (crosses), the best power-law fit (filled circles) and the best broken power-law fit (triangles). These residuals are shown in grey for the individual galaxies and in colour for the average residuals in each metallicity bin defined in Sect. 3.1. The colours show the number of galaxies in each bin. The mean residual for all of the observed G/D values is shown by the dashed (reference scaling), dotted (power-law fit) and dash-dotted (broken power-law fit) lines for the three relations and are reported in Table 1.