An alternative, instead of doing the computation within LAMMPS, is to produce dump files of the atomistic configuration including the velocity of each particle and then open it with OVITO (www.ovito.org) which is a very powerful tool that will allow you to plot velocity distribution among other features of your configuration.
thank you for your reply. slip velocity is the velocity of water near the wall or the mean velocity of all the water molecules in the z direction. is it possible to compute it with OVITO?