The gas-phase metallicity as a function of the gas fraction, using the four chemical evolution models. Top panel: all models are reproduced by adopting solar metallicity as y_eff (0.02). The closed-box, accretion, leaky-box (λ = 5), and gas-regulator (λ = 5) models are shown with the black solid, red dashed, magenta solid, and cyan dashed curves, respectively. Bottom panel: same models, but using the estimated y_eff and λ in Section 4.4 for Leo A. In both panels, the grey vertical bands show the estimated μ at 10 Gyr ago and the present day, whereas the horizontal bands show the stellar and gas-phase metallicities.