Observed astrometric variability of the photocenter, measured for SDSS J121855.80+020002.1 at z = 0.327 (up) and SDSS J162011.28+172427.5 (MRK 877) at z = 0.112438 (down), during the period 2007–2009 (see Table 3). The straight solid lines in the panels represent linear fits through the observed positions. Notations of points from 1 to 5 correspond to the observational epochs from first to the last as given in Table 3.