fix_ave/spatial command

Hi all,

I have a question on regarding how to use fix ave/spatial commend.

I have arranged manually arranged atoms as a 2D structure and submitted to lammps as a input file, and I'm pulling my 2-D structure x-axis (left and right) as shown below.
  <-- l l -->

I have used fix ave/time as the following

compute MyTemp all temp
compute MyPE all pe
compute peratom all stress/atom
fix ThermoAve all ave/time 1 100 500 c_MyTemp c_MyPE

If I want to obtain spatial average of the stress (I use velocity commands to apply stress) by using fix ave/spatial command, what do I need to do? what compute command do I need to give?

I think what I need to do is:
use compute command (something scalar) and then use fix ave/spatial command.

But I was not sure how to specify dimension, origin and delta variables to use fix ave/spatial command.