When object is dropped from to of the building then during it fall the net force on it is only due to gravity which act downwards
so we will have
[tex]F_{net} = mg[/tex]
now we have
[tex]ma = mg[/tex]
[tex]a = g = -9.8 m/s^2[/tex]
so during it's fall the net force force on the ball is constant and hence the acceleration is constant which is 9.8 m/s/s during its fall
Since there is no consideration of air drag so we have this value of acceleration to be constant through out the motion
and this graph will be a horizontal straight line for complete time
