Polarisation degree (top), Stokes parameters (middle), and polarisation angle (bottom) as a function of the relative intensity for the optical emission from J1023. The top panel shows the results of the Spearman rank correlation test and of a linear regression algorithm to the data. The shape of the anti-correlation is consistent with ΔP_opt / Δmag = -1.0±0.2, with an intrinsic random scatter around the best regression model of ${\sigma }_0=0.{07}_{-0.02}^{+0.04}$ (see text for details).