Two-dimensional histograms with background colours encoding the density of points on a logarithmic scale, showing p (top), 𝒮 (middle), and 𝒮 × p (bottom) as a function of the column density N_H (left), Galactic latitude b (middle), and Galactic longitude l (right). The resolution is 160′. The colour bar shown in the top left panel is common to all plots. Black curves show the running means calculated as in Fig. 12, with error bars representing the scatter in each bin. For 𝒮, which is on a logarithmic scale, the median trend shown (thin black line) follows the density of points more faithfully than does the mean (thicker black line).