Photocentre shifts of all used CO lines versus PA. Black and red symbols refer to ¹³CO(1−0) and ¹²CO(2−1) lines, respectively. Up and down triangles give the measurements for the blue and red line components, respectively. For each line and PA, two measurements are plotted corresponding to the two nodded spectra. The data for the ¹³CO(1−0)R21 line are connected by a line to guide the eye. The PA = 0° and 90° data of 2013 have been offset in PA by −3° for clarity. The PA = −60° data are not shown because their offset is not significantly different from zero.