Top left: observed CO SLED of PLCK_G244.8+54.9, the Ruby (black points), without correcting the velocity-integrated fluxes for gravitational magnification. The orange line shows the best-fit model with RADEX using a single gas excitation component, and illustrates a case where this simple model poorly reproduces the CO ladder for J_up ≳ 6. Top right: best-fit model from RADEX using two gas excitation components (solid orange line). The dot-dashed orange line shows the low excitation component, which is assumed to be cooler and more extended than the high excitation component (dashed yellow line) in the analysis. Bottom: one-dimensional and joint two-dimensional posterior probability distributions of n_H2, T_k and N_CO/dv, obtained from our MCMC sampling of the RADEX parameter space in the two-component model, for the low (left) and high (right) excitation components. Contours increase in steps of 0.5σ. Yellow and orange solid lines show the maximum posterior probability of each parameter, while dotted lines mark the ±1σ range in the distributions. The resulting parameter values are listed above the corresponding histograms.