Same as Fig. 4, but for the Polaris Flare field. Two-dimensional distribution function of polarization fraction p and column density N_H. The distribution function is presented in logarithmic colour scale and includes only points for which p/σ_p> 3. The dashed red line corresponds to the absolute maximum polarization fraction p_max and the solid red curves show the upper and lower envelopes of p as functions of N_H. The solid black line is a linear fit to the decrease of the maximum polarization fraction with column density at the high end of N_H (see Table 2 for the fitting ranges and fit parameters).