Three-dimensional velocities along the maser streamlines. The colored dots and arrows and the red axes have the same meaning as in Fig. 3. (Left panel) Expanding view of maser positions and proper motions in the NE region. The green and blue dashed curves are the fitted sinusoids to the B1 and B2 streamlines, respectively, whose parameters are reported in Table 2. (Right panel) Expanding view of maser positions and proper motions in the SW region. The red dashed curve is the fitted sinusoid to the R1 streamline, whose parameters are reported in Table 2.