Line-of-sight (l.o.s.) velocity of heliocentric distance for the galaxies in the sample (circles). The systems without a literature measurement of the l.o.s. velocity are assigned a null value, exclusively for the purpose of this figure, and are indicated as crosses. The colour-coding is based on the uncertainty in heliocentric transverse velocity, derived from the statistical uncertainties in the systemic proper motions and distance modulus (see Sect. 4).