Density–temperature (n − T) phase diagram of all six models used in this analysis. The rows show the different models, while the columns represent the different synthetic emission lines. The colours represent the emission in a particular line. For the MPM the densities do not directly correspond to those taken directly from the simulations, as they are given sub-grid density profiles, as described in Sect. 3.2. The bottom row corresponds to the fiducial RT model, and it shows how the RT post-processing produces densities and temperatures that differ from the original simulation.