Linear polarization state as function of time t and observation frequency ν. We give the result of each polarization measurement via a black bar in the t − ν plane. The length of each bar corresponds to the degree of linear polarization according to the scales in the upper left corners of the corresponding panels (in units of %). The orientation of the bar indicates the polarization angle; angles are counted counter-clockwise from the 12-o’clock position. Points in the t − ν plane where a measurement was carried out but no polarization was detected are indicated by gray bars. The length of each gray bar corresponds to a 3σ upper limit on the polarization level. As upper limits do not have meaningful polarization angles, all gray bars are fixed at the 12-o’clock position.