Cold gas mass against 0.5–7 keV X-ray luminosity within R₅₀₀ for a subset of our groups. Black points indicate systems in the CLoGS high-richness sample (O’Sullivan et al. 2017), for which complete X-ray data are available. Grey points mark additional groups for which X-ray luminosities were drawn from the literature. The dotted line indicates the 10⁴¹ ergs⁻¹ dividing line between X-ray bright and faint systems.