Newton's 2nd law of motion is: Force= (mass) x (acceleration).
Divide each side by 'acceleration' and you have: Mass = (force) / (acceleration).
-- At any point on the graph, read off the force and the acceleration.
-- Divide the force by the acceleration.
-- The quotient is the mass.
It should be constant, unless the moving object is something like a rocket,
which is constantly burning fuel and reducing its mass.