V/I line-ratio histograms (solid lines) with Gaussian fit functions (dashed curves) for the same observational sample that was used in Figs. 3 and 4, for |V/I| bins of 1.0 ± 0.9%. The positions of the Gaussians, marked by the vertical dotted lines, are much below the WFS positions, marked by the vertical dashed lines. Left panel: green line pair, representing a total of 731 data points. Right panel: red line pair, representing a total of 822 data points.