X-ray luminosity distribution as a function of the stellar bolometric luminosity. The three couples of lines refer to the (25%, 50%, 75%) quartiles of the L_X distributions calculated in running intervals of L_BOL constituted by 80 contiguous points and smoothed in intervals of bolometric luminosity of 0.1 erg s^-1. The continuous lines refer to Class II stars, the dashed ones to Class III stars. The quartiles have been plotted just where the fraction of upper limit values does not affect the calculation of the quartile itself.