Do you guys know what would be the curve looks like in the acceleration versus velocity graph? (including the air resistance when an object is falling)
Thank you!

That's a bit of a weird graph to draw, but here you go:

This is assuming that the force due to air resistance is proportional to speed, but it might actually be the square of speed or something.

thank you for showing.
but doesn't it look like a straight line or a curve? And also I am wondering a vs v, which is acceleration as y-axis and velocity in x-axis.
thanks

Hmm, it would be a curve because it is proportional to the speed squared. You can flip the axes around easily.

There are some useful specifics on this page:

Drag (physics) - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Air_resistance)

That DO answer your question :)

Acceleration vs velocity... but what is the units for the gradient of that graph?

$s=ut+\frac {1}{2}at^2$ this is displacement of an accelerated objects