Velocity offset of the XMP galaxies as a function of the best-fit radial velocity and spatial offset. The H i, Balmer emission-line and forbidden emission-line velocity offsets, Δv_H   i, Δv_Balmer and Δv_forbidden, respectively, are the displacements of the H i systemic radial velocity, Balmer lines and forbidden lines with respect to the best-fit radial velocity, v_z (Sect. 3.3 and Table 5). Only non-zero values are plotted. The color code parametrizes the morphology as follows: red = symmetric, black = cometary, blue = two-knot, green = multi-knot and grey = no morphological information. The symbol code is •= Δv_H   i, □= Δv_forbidden and △  = Δv_Balmer. The magenta line defines the null velocity offset.