res yoursavfile
config gpextra 2

def _transfer
  _g = gp_head
  loop while _g # null
    gp_extra(_g,1) = 0
    gp_extra(_g,2) = 0
    _g = gp_next(_g)
  end_loop
  _z = zone_head
  loop while _z # null
    if z_model(_z) # 'null' then
      _szz = z_szz(_z)
      _vol = z_volume(_z)
      _ingp = z_numgp(_z)
      loop ii(1,_ingp)
      _g = z_gp(_z, ii)
      ; do a volume weighted average of sigma zz
      ; and store result at the gp extra location
      gp_extra(_g, 1) = gp_extra(_g, 1) + _vol
      gp_extra(_g, 2) = gp_extra(_g, 2) + _szz * _vol
      end_loop
    end_if
    _z = z_next(_z)
  end_loop

  _g = gp_head
  loop while _g # null
    if gp_extra(_g, 1) > 0 then
    gp_extra(_g, 2) = gp_extra(_g, 2) / gp_extra(_g, 1)
    end_if
    _g = gp_next(_g)
  end_loop
end
_transfer

plot cont gpextra 2 null on outline on shade on
ret