Observed Stokes values of θ UMa. Upper: Stokes values after subtraction of the artificial polarization. The colors represent each of Stokes: ${\hat{q}}_1$ (blue), ${\hat{u}}_1$ (orange), and ${\hat{v}}_1$ (green). The shapes of the plots correspond to observation dates as explained in the lower panel. Fit curves in the form of Eq. (12) are also drawn. Lower: differences between the observed values $({\hat{q}}_1,{\hat{u}}_1,{\hat{v}}_1)$ and the fit curves.