Same as for Fig. 1, but for 966 red giants from Zhang et al. (2020) using LAMOST data. The best linear fit corresponds to a coefficient a = 1.64 ± 0.01. Stars with a relative deviation below 10% between S-index measurements are represented in blue, with error bars in gray for visualization purposes. Stars with a relative deviation of at least 10% between S-index measurements are represented in red, as well as the associated error bars.