How do you calculate the gravitational potential energy of a 2 kg bottle of soda falling?

If the bottle is falling, then its gravitational potential energy is also falling.  That's because gravitational potential energy is  (mass)·(gravity)·(HEIGHT) .  

So the best I can tell you is that at any instant of time, the bottle's GPE is

(2 kg) · (9.81 m/s²) · (HEIGHT above ground at that instant) .

That's going to be  (19.62 x height above ground in meters), and the unit, of course, will be 'Joules'.

  distance on the y-axis and time on the x-axis.

explanation:

the instantaneous velocity = change in position/time taken

the slope of the graph should give instantaneous velocity.

  slope of a graph = change in value of y -axis / change in values of x -axis

  comparing both the equations

    change in value of y -axis = change in position

  change in values of x -axis = time taken

so position values should be on the y axis and time values should be on the x axis.

distance on the y-axis and time on the x-axis.