Polarization angle χ as function of the square of the rest-frame wavelength λ₀. Polarization angles are restricted to the interval  [0°,180°] ; for 0954+658 and 1637+574 these intervals have been shifted to resolve 0°/180° ambiguities. The selection of data is limited to the epoch ranges quoted on top of each plot to maximize the significance of a rotation measure signal. We quote correlation coefficients r and significance levels s like in Fig. 3. For 0415+379, 0507+179, and 0954+658, we derive 3σ upper limits on |RM|. For 1418+546 and 1637+574, we quote the rotation measures together with their statistical and systematic errors. Rotation measures are derived via linear fits (gray lines) to the data.