Bulk Lorentz factor as a function of distance z from the black hole. The distance is displayed in units of R_g (top axis) and milliarcsecond (bottom axis). Colors indicate observations at different bands from various monitoring programs. Error bars represent ±1σ uncertainties. The solid line represents the best-fit broken power-law model. One of the knots traced at the X-band (X3), marked by an empty colored cross, is an outlier from the general relation. The vertical dashed line indicates the location of the transition point of the jet kinematics, $z_{\mathrm{b}}^{\mathrm{\Gamma }}$.