Chromospheric emission index versus the inverse of the planet gravity (in cm^-2) with the two best-fit regression lines of the mixture model, described in Fossati et al. (2015a), in black and red. The data points assigned to each of the two regressions are plotted with the same colour coding of the corresponding regression line. The high- and low-activity components have respectively an intercept of −4.47 ± 0.14 and −4.95 ± 0.06 and a slope of −501 ± 247 and −44 ± 54 cm s^-2.