S_​HK and R_X time series for HD 166620 with data compiled by Luhn et al. (2022) and more recent data from Keck/HIRES (Isaacson et al. 2024) and the APF (Vogt et al. 2014). The small transparent symbols mark individual S_​HK measurements, and the larger opaque symbols are the same data, binned in 120 d bins. A black outline denotes a bin containing only a single measurement. The dashed blue line shows the extrapolated trend of the two maxima (see Sect. 2.1). The two X-ray measurements are overplotted with black squares, and their epochs are marked on the axis.