explain how to plot magnetic field of bar magnet by a compass ?

We move the compass near the magnet and observe the orientation of the needle. Now, when we move the compass in the direction indicated by the needle, we observe that the direction of the needle changes, until we reach one of the poles of the magnet. We trace this path followed and the path represents a magnetic field line. We do this over and over at different distances to obtain a number of such lines.