Spectral variation of optical lines. Upper panel: mean spectra near the lines Ca II H, Hβ, Fe II 4924 Å, Fe II 5018 Å, and Hα. Middle panel: differences between the high-activity (blue line) mean spectrum and the mean reference spectrum and the low-activity (red solid line) mean spectrum and the mean reference spectrum. Lower panel: periodogram height at P = 3940 d for each spectrum pixel. Horizontal dashed lines correspond to FAP = 10⁻⁵, 10⁻¹⁰, and 10⁻¹⁵ (see text for explanation).