In a projectile motion let $v_x$ and $v_y$ are the horizontal and vertical components of velocity at any time $t$ and $x$ and $y$ are displacements along horizontal and vertical from the point of projection at any time. Then

If $u$ is the initial speed and $\alpha$ the angle of projection. Then $v_y=u \sin \alpha -gt$  i.e., $v_y-t$ graph is a straight line with negative slope and positive intercept. $x=(u \cos \alpha )t$i.e., $x-t$ graph is a straight line passing through the origin. $y=(u \sin \alpha )t-\frac{1}{2}g{t}^2$ i.e.,$y-t$ graph is parabola i.e., $v_x-t$ graph is a straight line parallel to $t-$axis.