Stokes parameters Q (top) and U (bottom) versus total intensity signal in the Cygnus X star-forming complex. With different colours we represent WMAP 23 GHz (green) and Planck-LFI 30 GHz data for three different cases: PR3 raw (un-corrected) data (red), PR3 leakage-corrected data (blue) and PR4 leakage-corrected data (gold). In the legend we quote the Q/I and U/I polarisation fractions derived from the linear-regression fits represented by the solid lines (same values that are quoted in Tables A.2 and A.1.).