Graphical representation of the Mueller matrix of a non-ideal HWP. Each panel corresponds to the profile of a matrix element as a function of frequency. We used the simulated profiles of h and β for the MFT bands, while we fix ζ_1, 2 = 0.01 and χ_1, 2 = 0. The black dashed line represents the case of an ideal HWP, the shaded vertical bands correspond to the four MFT bands, labeled by their central value. The non-diagonal blocks do not vanish as a result of ζ_1, 2 ≠ 0 (see Eq. A.2).