Age−metallicity relation for GCs with known rotation properties. The normalised ages are taken from Marín-Franch et al. (2009) based on the Zinn & West (1984) scale. The points are colour-coded based on the A_rot/σ₀ ratio. Slow rotators are marked with red and fast rotators with blue. NGC 4372 is marked with a red star. The two sequences of GCs supposedly accreted into the Milky Way halo and born in-situ are marked with black lines.