Scatter plots of the variation of the Stokes Q and U parameters across the HBA band, calculated for the two pulse phases of PSR B1919+21 that correspond to a) the maximum (−15.77(1) rad m^-2) and b) the minimum value (−16.61(3) rad m^-2) of the first derivative of the PA with respect to λ² (middle plot; the selected phases are highlighted with grey circles). The expected periodicity of the Stokes parameters with frequency, based on the above values of ∂PA /∂λ², is shown for each case with a red curve. The residual difference between the expected variation and the data is plotted below each of the Stokes Q and U plots, where the dashed red line corresponds to zero residual difference.