Left panel: close-up view of the F_HK vs. F_Hα diagram. Colors code the metallicity of the stars as shown in the legend. Black crosses represent the typical measurement uncertainties at different activity levels. Middle panel: same as in the left panel for the median of the flux measurements. For each star, the error bars represent the MAD of the measurements shown in the left panel. Right panel: the same as in the left panel, representing each star with the corresponding linear fit to its flux-flux values. Gray lines mark best fits with low statistical significance, while black lines represent p-values lower than 1% based on Spearman’s correlation test.