A plotting compass is a very small magnetic compass. Its needle is a tiny magnet that is free to turn, so it lines up with the magnetic field at its position. The north-seeking end of the needle points in the direction of the field.
To plot the field of a bar magnet, place the magnet in the middle of a sheet of paper and draw round it. Put the compass near one pole of the magnet and mark a dot at each end of the needle. Move the compass so that the end of the needle that was at the back now sits on the dot you marked at the front, then mark a new dot at the front end. Repeat this to make a trail of dots, then join the dots with a smooth line. Add an arrow pointing the way the north end of the needle points. Start again at different places around the magnet to build up the pattern of field lines.
The same compass shows the Earth's field. With no magnets nearby, the needle points towards the north. This shows that the Earth has a magnetic field, and that the needle turns to line up with it.
