Ratios of global mean periods (P_i/P_i + 1, i = 1, 2, …) of M1, M2 and the total mean periods shown in the panels of Fig. 2. The formatting of the data points and horizontal lines is the same as in Fig. 2. The colouring in each panel coincides with the period standing in the denominator in each ratio. On the vertical axes, the levels of the period ratios in the reference spectrum are labelled and the corresponding horizontal solid black lines are plotted in all panels.