X-ray surface flux of all stars with known X-ray cycles as a function of the cycle amplitude, i.e. the variation of L_X between the maximum and the minimum of the activity cycle. Kepler-63 is included in the plot, although we detect no X-ray cycle. The colour bar denotes the ages of the stars. The dotted red line is the linear regression performed on the data set. The result of the fit is given in the legend of the plot, while the residuals of the procedure are reported in the bottom panel.