Motion With Uniform Velocity 

Consider a particle moving along x-axis with uniform velocity u starting from the point x =x_i at t = 0.

Equations of x, v, a are : x (t) = x_i + ut ; v (t) = u ; a (t) = 0

x- t graph is a straight line of  slope u through x_i.

as velocity is constant, v - t graph is a horizontal line.

a-t graph coincides with time axis because a = 0 at all time instants.