Measured excess flux in the Ca ii H and K lines as a function of S_MWO. This plot shows data from 2154 observations of 80 stars. For reasons of clarity, we have removed HD 22468 from this plot, as it contributes a number of datapoints clustering around a value of S_MWO ≈ 1.25, as well as some observations with a negative value of F_Exc,HK that is consistent with zero considering its error.