Upper panel: TSS (slope of the differential velocity shifts of spectral lines, in m/s/(F/Fc)) averaged for each sample versus the average temperature of the sample (red filled circles and solid line). The green curve (open circles and dashed line) shows the TSS after the wavelength dependence correction. Middle panel: same for the slope of the TSS versus Log . Lower panel: same for the ratio between this slope and the TSS. The dotted line is a linear fit on the ratio in red.