Scatter plot in the plane (cos(Δ_S/V), R_P/p) and binned data. The error bars correspond to the errors on the mean of cos(Δ_S/V) in bin that are due to experimental uncertainties on polarization position angles (dust and quasars). Sampling errors in bin are about two times larger. Vertical gray lines show the borders of each bins. They were chosen such that each bin contains 51 data points, except the last one which contains 49. The horizontal tick line shows the expected mean under the assumption of uniformity in bin and the horizontal dashed lines show the one standard deviation from this mean, computed for 51 data points.