Histograms of the linear polarisation fraction (top rows) and polarisation angle dispersion function (bottom rows) for six different cuts in galactic coordinates, as indicated above each panel. These histograms were computed for the masked images shown in Figs. 2 and 3 that use our reference dust model with alignment fraction f_S, A = 0.6. The different colours correspond to different observer positions (columns in Figs. 2–3), while the different line styles correspond to the different observer densities (rows in Figs. 2–3). The opacity of the lines is determined by the dust allocation recipe, as indicated in the legend. The black solid line corresponds to histograms computed from the publicly available Planck HFI 353 GHz map, as presented in Planck Collaboration XII (2020), using the same cuts in galactic coordinates. All histograms show the actual pixel count; no normalisation is performed.