Variation of the CO flow axis inclination to the line-of-sight i derived from the observed variation of assuming that the gas flows along the conical surface of semi-opening angle θ = 35^∘. Plotted error bars include the 1σ uncertainty on V_lsr and θ. The dashed green curve shows the prediction from the precession model that best fits the centre position and centroid velocity wiggling of the CO flow axis. See Sect. 5.1.3 for more details.