Correlation between peak separation and maximum velocity difference measured from double-peaked spectra. The peak separation is defined as the difference between the wavelengths of k_2v and k_2r in units of the Doppler velocity. The maximum velocity difference, max(Δv_z), is defined as the difference between the maximum and minimum velocity in the atmosphere between the formation height of the central minimum k₃ and the minimum of the k_2r and k_2v formation heights. The blue, green, and red contours enclose 90%, 50%, and 25% of the dataset, respectively. Both quantities are measured from the non-degraded spectra. The gray scale of the pixels in the 2D histogram is saturated at 0.8% to increase the readability. The top and right panels show the marginal distributions as a percentage.