The intrinsic polarisation of the jet emission P₀ derived in Sect. 3 versus the jet flux density in the R band. We have assumed an observed brightness R_QSO = 17.85 to correct for the QSO-like unpolarised emission contribution that affects both the flux and polarisation degree observed. The blue lines represent the results of a shock-in-jet model where the direction of the front wave, hence the polarisation degree, vary because of the variation in the viewing angle. We adopted an optical spectral index of 1.6, as found in Sect. 3. Values of the shock wave degree of compression η in the 1.15–1.7 range can account for most of the data dispersion.