Physics velocity graph question

The axes are vertical velocity (y) against horizontal velocity (x). As you say the horizontal velocity is constant so ... The vertical velocity is changing while the horizontal is constant. The gradient here is "infinite" (really its indeterminate), not zero.

Ahh, so the stone is still accelerating vertically right and the infinite gradient is the acceleration?

No, you cant say anything about the vertical acceleration, apart from its not zero (so the vertical velocity changes). Acceleration is the rate of change of velocity and you need to know
change in velocity / change in time
You dont know anything about the change in time from this graph.

All you can say from the graph is that vertical velocity is positive downwards.

But would the weight of the stone mean it’s accelerating and air resistance is negligible?

The vertical velocity is increasing (positive downwards) so from the graph its certainly accelerating. And basic physics would tell you its accelerating at "g", but you cant determine the acceleration value from the graph.

Ahh ok thank you. So the gradient of the graph says it’s accelerating because the gradient is incite but we cannot determine the acceleration?

You seem hung up on x-axis being time so the gradient represents dv/dt which is the acceleration.

Here the x-axis is horizontal velocity and it does not matter what the shape of the graph is, you cannot determine the vertical acceleration from such a graph. Here it simply says that the vertical velocity changes between 0 and a maximum value and the horizontal velocity is constant. Its meaningless to talk about its gradient for a vertical line and even if you could (the graph wasnt a vertical line) it does not correspond to (vertical) acceleration.