Illustration of the recalibration method. Top panel: raw spectra show significant shape disparities, probably resulting from changes in instrument calibration throughout the years. These calibration disparities are hardly fitted by polynomials. Centre panel: zoomed-in view of 2680–2820 Å region, showing the splines fitted to each spectrum. The grey areas indicate the different spectral domains where the reference fluxes were measured (black crosses), which are then used to calculate a cubic spline for each spectrum. Bottom panel: zoomed-in view of 2735–2753 Å region after renormalisation by the cubic splines. The spectra taken at different epochs are shown with different colours; they are well superimposed in the region of the stellar continuum, while variable absorptions are clearly visible around the different Fe II lines between 2737 and 2750 Å.