Propagation speed measurement. Top panel: lag values as a function of the time-series position. The zero time-series number corresponds to the middle time-series in the range. A linear fit is shown by the red line, the gradient of which determines the propagation speed. Bottom panel: typical distribution of time lags between two time-series from the re-sampling technique. The red line is a fitted Gaussian with the mean shown by the dashed red line. The mean and sigma values of the Gaussian are used as the lag and uncertainty on the lag respectively.