Watch a bottle and a piece of drift wood on the water surface. The wave is traveling from the bottle to the wood. If you draw a displacement vs distance graph , you would be locating the displacement of the bottle and wood in their respective distance from each other. If you draw the displacement time graph , you can only draw for either the bottle or the wood as it bobs up and down in its respective distance. To know the direction of the velocity of the bottle or the wood at any specific time , you would just look at a later time , i.e to the right of the x-axis. To know the direction of velocity in the displacement vs distance graph , for the wood or the bottle , you would look opposite to the direction the wave is traveling , i.e towards the left of the x axis( wave traveling left to right ).If the wood and bottle is next to each other , the wood will assume the bottle displacement later in time , so just look towards the bottle.
