Upper panel: the evolution of the linear fit χ²/d.o.f. as a function of the fitted line orientation when constant polarization is assumed for SN 2007uy. The minima correspond to the optimal orientation of the straight lines displayed in Fig. 1 bottom panel, also given in Table 3. As seen the global shape of the evolution for the three independent data-sets (VLT, NOT and CAHA) are quite equivalent, their minima being systematically located at similar negative angles. The long-dashed line shows the χ²/d.o.f. evolution when all the data are jointly considered. We note that in the case of CAHA, given that d.o.f. = 0, χ²/d.o.f. actually represents χ². Bottom panel: the evolution of the normalized probability associated with the above panel. As displayed the three independent data-sets show their probability maxima at a comparable orientation angle. The maximum is reinforced when all the data are combined (long-dashed line). Given that d.o.f. = 0 for CAHA, the corresponding probability distribution was obtained through a Monte Carlo method. We refer the reader to the main text for further information.