F-t graph

In the upward journey, the net force acting on the block Fu is,
Fu = -(mg.sin(a) + Ff)
where m is the mass of the block
g is the acceleration due to gravity
a is the angle of the inclined plane
Ff is the frictional force.

Because both mg.sin(a) and Ff are pointing downward parallel to the slope, Fu is -ve. Hence, the F-t graph is a straight line, with ordinate (i.e. value on y-axis) equals to -(mg.sin(a) + Ff), parallel to the time axis (t-axis)

When the block moves downward, the net force Fd is,
Fd = -mg.sin(a) + Ff
Now, the frictional force, which is always in opposite direction to the motion of the block, points upward along the slope.
Hence, Fd = -(mg.sin(a) - Ff)

Thus, Fd < Fu in magnitude.
The graph will be a straight line parallel to the t-axis, but with absolute value less than that in the upward journey.

The "step" on the graph is the time when the block reaches the highest position on the slope.