Slopes between averaged indices for stars in the low-activity regime versus T_eff (upper panel), and the number of stars in each T_eff bin (lower panel), corresponding to the three pairs of indices. Slopes correspond to Ha versus Ca (black), Na versus Ca (red), and Hα versus Na (green, divided by 10 for clarity). The number of star per bin in black corresponds to the Hα–Ca and Na–Ca pairs, while the number of stars in green corresponds to the Hα–Na pair.