Multiplicative vs. additive ridge alignments. The upper panels show the result of the exact Skumanich-type period scaling with , where t stands for cluster ages. The scaling factors are displayed in the boxes. The lower panels show the same test with the additive scaling of P_rot → P_rot + c, where c is the period shift as shown in the corresponding boxes. The fiducial ridge (see Eq. (3)) is plotted as a light color continuous line.