How do you find an object's mass on an acceleration vs. force graph?

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.