Reconstructed evolution of the on-sky linear Stokes parameters in QU space for the July 28 flare, linearly interpolating to fill in U′ and Q′ where the other is measured. Colour indicates time in minutes. Left: previous calibration where the quantities have only been subjected to a field rotation correction (Gravity Collaboration 2018b). Right: full new calibration including VLTI systematics and Stokes V′ reconstruction. In both cases, the flare traces 1.5 loops during its 60 − 70 min evolution.