Velocity difference (∆ V) versus V_obs. Different symbols correspond to spots at different distance intervals. The small crosses are for d_cc < 0.09 pc, the dashed line is fitted to these spots. The small circles are for spots at 0.09 pc < d_cc < 0.12 pc, and the dotted line is fitted to them. The continuous line is fitted to spots at 0.12 pc < d_cc < 0.19 pc, which is shown with mid-size crosses. The large circles are for 0.19 pc < d_cc< 0.25 pc and large crosses for 0.25 pc < d_cc < 0.42 pc.