Cluster speed relative to the motion of the progenitor clusters of each chain, plotted against the time of cluster formation. We estimate the average acceleration along each cluster chain with linear regression curves (solid gray lines), including two alternative fits to the data (dashed light-gray lines). See Sect. 3.2 for details. We list the gradients in Table 1 and all fitting parameters in Table A.3. In the bottom panel, we indicate the two subchains Upper-Sco I and Upper-Sco II with diamonds and squares, respectively.