Histogram of the X-ray luminosity of AGB stars and SySts. Previously known X-AGB stars are shown with black crossed-dashed symbols, newly found X-AGBs are shown with black diagonal-dashed symbols. The X-SySts are shown with colors, according to the classification of their X-ray spectra, as labeled. The dotted and solid histograms are the best least-squared Gaussian fits to the X-AGBs and X-SySts, respectively.