Variation of the V/I line ratio with V/I amplitude, as determined for each separate |V/I| bin, for the same observational sample that was used in Figs. 3 and 4. The left panel represents the green line pair, the right panel the red line pair. Due to the steep increase of the curve toward the WFS level in the limit of small |V/I|, a logarithmic scale is used for the horizontal axis.