load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"  
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"  

begin
  wks = gsn_open_wks("x11","poly2_ex03")

  res           = True                        ; plot mods desired

  res@gsnMaximize = True                      ; maximize plot in frame
  res@gsnFrame    = False                     ; don't advance frame
  res@gsnDraw     = False                     ; don't draw plot yet

  res@mpMinLatF   =  -20                      ; zoom in on a subregion
  res@mpMaxLatF   =   40
  res@mpMinLonF   = -130
  res@mpMaxLonF   =    0

  map = gsn_csm_map(wks,res)

;---Create list of lat/lon points that will form a box.
  boxlat = (/ 30.0,  30.0,  0.0,   0.0, 30.0/)
  boxlon = (/-90.0, -45.0,-45.0, -90.0,-90.0/)

;---Attach the box
  gonres                  = True              ; polygon mods desired
  gonres@gsFillColor      = "brown"           ; color of polygon
  gonres@gsFillOpacityF   = 0.5               ; half transparent

  dum = gsn_add_polygon(wks,map,boxlon,boxlat,gonres)      

  draw(map)
  frame(wks)
end