Mean photocentre shifts of all used CO lines versus PA. Colours and symbols as in Fig. 4. The PA = 0° data of 2013 have been offset in PA by −3° for clarity, the PA = 90° and −60° data are not shown because their offset is not significantly different from zero. The blue, green, and orange lines show the predictions from the simulations (see Sect. 5) for three different combinations of blob distance and flux ratio.