CO excitation diagram towards three CO absorption systems. The grey, red, and violet points indicate the column densities values sampled from the posterior distributions of the fit parameters using the model described in Sect. 2 for SK −67 2, SK −68 137, and SK 143, respectively. The stripes of the corresponding colours indicate the 0.683 credible region of the excitation diagram derived from the fit to the sampled column densities with Boltzmann law.