Distribution of β_mm as a function of τ₃₅₃, for the whole region. The red line gives the best linear fit for τ₃₅₃ ≥ 4 × 10^-4 (see text). The colour scale is logarithmic and it represents the density of points. The three contours show the densities for a cumulated fraction of 11, 31, and 62% of the data points, from red to yellow.