Rotation strength, taken from Leitinger et al. (2025), as a function of the flattening we measured. The names of GCs are indicated close to the corresponding points and coloured according to the inclination angle of the cluster, also taken from Leitinger et al. (2025). Low inclinations are plotted in blue while high inclinations are coloured in yellow. The uncertainties associated with the inclination angles are not shown in this figure. Instead, we report them in the last column of Table C.1. The dashed lines indicate the relation for isotropic oblate rotators viewed at different inclination angles.