Observations of UP stars. Upper: raw Stokes values plotted against angles of the instrumental rotator (ρ). Blue circles, orange triangles, and green squares represent ${\hat{q}}_{\text{raw}},{\hat{u}}_{\text{raw}}$, and ${\hat{v}}_{\text{raw}}$, respectively. Fit curves in the form of Eq. (12) are also displayed. The data points shown as open plots are excluded from the fitting. The parameters of the fit curves are summarized in Table 2. Lower: differences between the raw Stokes values and the fit curves. The RMS values are written at a corner.