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 Zhukovska (2014). The symbols are the same as for Fig. 4. The model from Zhukovska (2014) is shown for various star formation histories: episodic with 6 bursts of 50 Myr and star formation timescale of τ_SF = 2 Gyr (orange), episodic with 6 bursts of 500 Myr and τ_SF = 2 Gyr (green), episodic with 6 bursts of 500 Myr and τ_SF = 0.2 Gyr (more intense star formation, brown) and continuous with a star formation timescale τ_SF = 10 Gyr (cyan dash-3 dots line). 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.