Speed-time relation. Relative cluster speed (v) versus lookback time (t), with the oldest cluster (e Lup) as a reference point that is excluded from the linear fit (dashed black circle). The symbols are colour-coded by formation time (or cluster age; see the x-axis) and scaled by number of sources per cluster. The linear fit (solid grey line) was obtained via bootstrapping, with the median fitting parameters given in the upper left corner. The fit uncertainties are plotted as shaded grey areas (1-2-3 σ).