energy problem

Since you haven't state the conditions, I assume that the box starts from rest and the slope is smooth, i.e. frictionless.
Using equation of motion: v = u + a.t
now, u = 0 m/s, a = g.sin(p), where p is the angle of slope
thus, v = [g.sin(p)].t
Kinetic energy KE = (1/2)mv^2 = (1/2).m.[gsin(p)]^2.t^2
where m is the mass of the box
therefore, KE is proportional to t^2, the KE-time grapg is a parabolic upward increasing curve.
Using equation of motion: v^2 = u^2 + 2.a.s
here u = 0 m/s, a = g.sin(p)
v^2 = [2.g.sin(p)].s
KE = (1/2)m.v^2 = (1/2).m.[2g.sin(p)].s = [mg.sin(p)].s
hence, KE is proportional to s, the KE-displacement graph is a straight line with positive slope.