Optical photometry and polarimetry and γ-ray light curve of 3C 279. Fermi-LAT γ-ray light curve at > 100 MeV binned into 3 day intervals (panel a)) as published in Hayashida et al. (2015). Combined R-band light curve (panel b)). Measured, optical polarization fraction (panel c)) and EVPA (panel d)); red circles: Calar Alto (R), red squares: CrAO-70 cm (R), red diamonds: Perkins (R), orange up-sided triangles: SPM (R), orange right-sided triangles: St. Petersburg (R), green down-sided triangles: KANATA (V), green left-sided triangles: Steward Obs. (spec. and V), blue circles: Liverpool (V+R), blue squares: KVA (white light). Combined, de-biased, and averaged polarization fraction (panel e)). Combined, averaged, and adjusted EVPA (panel f)); open symbols are added from the non-averaged EVPA curve. Pointwise, local derivative of the adjusted EVPA (panel g)). The grey area highlights the period of γ-ray flaring activity coinciding with a rotation of the optical polarization angle.