Correlation between line depth and υ_rad. TO data are marked with circles and FIES data with stars. The grey areas describe the correlation between υ_rad and line depth during two distinct time periods: Area no. 1 is for observations made between March 2023 and Aug. 2023; Area no. 2 is for spectra from Dec. 2023 onwards. Highlighted dates show different correlation behaviour. The orange-coloured dates are from the time period between those covered by the grey areas: the correlation shift from area 1 to area 2 happened within a few months in late 2023. The blue-coloured dates are from the dimming minimum, when lines were extremely shallow.