How to plot stress relaxation data

Hi,

I've implemented a confined compression test on a cylindrical disk using the poroelasticity model with a isotropic solid matrix. The test consist in one ramp of increasing displacement (corresponding to piston drop) lasting 4 seconds during which the disck is compressed at 4% of its initial thickness , and then allowed to relax for 600 seconds by a maintaining the diplacement imposed.
The analitical function of the ramp imposed is written below:
((h1-h0)*(t/(t0+temp*0))+h0)*(t<(t0+temp*0))+h1*(t>=(t0+temp*0))
where:
h0= initial thickness;
h1= thickness after the first ramp of deformation;
t0= 4 s;
temp= 600 s.

My desire is to extract the stress-time plot in z direction.

I have done the following passages:
"Derived Values-> Line Average"( on lower sample boundary )
In this way the program shows only the plot of the stress-relaxation an not the one of the increasing ramp of the stress, although the diplacement step is correctly plotted.

Thanks a lot in advance,
Sara