G/D as a function of metallicity for the 2 values of X_CO: X_CO,MW (left) and X_CO,Z (right) with the chemical evolution model of Asano et al. (2013a). The symbols are the same as for Fig. 4. The colours delineate ranges in star formation timescales τ_SF. The model from Asano et al. (2013a) is overlaid on the points for various τ_SF = 0.5 (red), 5 (blue), 50 (purple) Gyr. The black dashed line represents the reference scaling of the G/D with metallicity (not fit to the data). The black dotted and dash-dotted lines represent the best power-law and best broken power-law fits to the data.