Number of stars (vertical axis) older than 9 Gyr with respect to the total number (annotated as text) of α-poor, rich stars (respectively) in a given [Fe/H] bin. Grey dotted lines represent [Fe/H] bin boundaries. Blue data represents α-poor stars, red data represents α-rich stars. The solid lines represent the left y-scale: fraction of stars with ages (τ) more than 9 Gyr, the dashed lines represent the right y-scale: fraction of stars with redshift (z) more than 2.