Left: the squared radial displacement versus time for all types of particles (τ_s ≡ t_stopΩ) in run tc = 10 (t_coolΩ = 10). The colour symbols (with solid curves) are measurement, the dashed curves are the linear fits to the data. Their slopes are then twice the diffusion coefficients based on equation (10). Right: symbols mark the radial diffusion coefficients for particles with different stopping time. At larger stopping time, the measured coefficients are largely different than models of either Youdin & Lithwick (2007) (dashed grey curve) or Cuzzi et al. (1993) (dotted) due to extra stirring from the gas self-gravity.