Average S_Hα versus average S_Ca for quiet stars. Stars with metallicity lower than −0.2 are in red, stars with metallicity between −0.2 and 0.2 are in black, and stars with metallicity larger than 0.2 are in green. Open symbols are subgiants. The small orange dots are nightly observations for stars with a correlation below −0.5. Each panel corresponds to a different T_eff range.