C¹⁸O luminosity versus ¹³CO luminosity for the DALI models (colored circles) and the observations (black circles). The colors indicate the gas mass of the disk model. The size of the markers indicate the 90% radius of the ¹³CO emission. Probability density curves of the models for each gas mass are shown along the x-axis in the top panel and along the y-axis in the right panel, i.e., most models of a particular gas mass reside around the peak of each curve.