The 84 VLTI/AMBER φ_diff(λ) measured on Achernar around Br γ at 28 different observing times (format YYYY-MM-DDTHH_MM_SS) and, for each time, three different projected baselines and baseline position angles, as indicated in the plots. The dashed gray horizontal lines indicate the median ±σ_φ =  ±0.6° of all observations. The smooth curves superposed to the observations are the best-fit φ_diff obtained with a uniform-rotation, gravity-darkened Roche model, as described in Sect. 4. All the observed φ_diff points are shown here, even if the fit has been performed using only each third wavelength point (cf. Sect. 4). All φ_diff curves are equal to zero in the continuum, but they were shifted for better readability. The other panels are available in the electronic edition.