2D histograms showing the correlation between RM^H (see Fig. 3), and the maps of the LOS-average magnetic field (blue) and the electron column density (red). The ordinate axis shows the corresponding values normalized to their 99th percentile. From light to dark colors, contours correspond to number of pixels of 50, 100, 500, 1000, 2000. Person correlation coefficients (R_p) are written. As an example, we show case A integrated along the y axis.