Corner plots of spectral fit parameters in chosen ROIs. In each 2D plot, the joint posterior distributions are shown, colour-coded by the likelihood, with red, orange, and yellow showing the 1, 2, and 3σ regions. On the diagonals, the marginalised PDFs of each parameter are shown. The solid blue line marks the expectation (=fit) value for the parameter, and the dashed blue lines the 68.3% intervals. We want to point out the natural anti-correlations between the continuum and o-Ps amplitude (C₀ vs. O₀ ), and between the line amplitude and width (A₀ vs. σ).