Hi Rafael,
You have to know that the pressure is normalized by the volume of your
system, as described in lammps' manual. In your case, P, Pxx, Pyy and
Pzz are all divided by the volume of your simulation box which is larger
than the volume of your graphene sheet. So you have to rescale the
pressure by the "real" volume of your graphene sheet. How to define the
"volume of graphene" is up to you.
Cheers,
-felipe.