Fittings of the (XX − YY) /I intensity ratio, to determine polarization percentages and polarization position angles. Data presented in this figure are self-calibrated ALMA band 3, 6, 7 observations of NRAO530 on UTC 2012 May 18. We fitted the four spectral windows of the band 3 observations together to enhance the signal to noise ratio. The best fits of polarization percentage (P), polarization position angle (in the receiver frame, i.e. Ψ − φ; PA), and a constant normalized Stokes Q offsets (Q offset), are provided in the upper left of each panel, which are represented by a black curve. For each observed frequency, errors of fitted quantities were determined by one standard deviation of fittings of 1000 random realizations of noisy data (details are in Sect. 3.2). Gray lines in each panel plot every 100 of the random realizations.