l.o.s. accelerations measured for individual 6.7 GHz maser features. Each panel shows a different cluster, from “A” to “E”. For each maser feature, the value of the l.o.s. acceleration is quoted in km s^-1 yr^-1. Symbols, colors and contours have the same meaning as in Fig. 1. In panels of the maser clusters “A”, “B” and “C”, the dashed line indicates the major axis of the maser feature’ spatial distribution, and the star marks the reference point to measure axis-projected offsets (see Sect. 4.1).