Bivariate kernel density estimate of the fraction of CO-dark H₂ gas f_dark vs. the [C II]/CO ratio for all pointings. The shade scales with the density of points. The dashed curve is an attempt toconnect most of the distribution peaks from different pointings (with the plots ordered according the location of probability peaks along the curve) while the solid curve shows the expected relation between f_dark and [C II]/CO at low density (~30 cm⁻³; Fig. 11).