If you are talking about 2d projectile motion with distance v. time for any fixed angle above the horizontal then the graph will be a parabola because distance is quadratic in time i.e. d = v(initial)*t + 1/2*a*t^2.
Because the horizontal velocity of the projectile is constant and the vertical velocity continuously decreases because of gravity and hence the distance the projectile traveled in a given time keeps on decreasing and horizontal distance remains the same. This results in a parabolic trajectory.
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