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

begin 
  npts    = 500                      ; number of points
  ncurves = 4                        ; number of curves

  ym = asciiread("xy2.txt",(/npts,ncurves/),"float")
  x  = fspan(-50,50,npts)         ; npts values from -50 to 50
  y  = ym(:,0)                    ; Just grab one set of points.
 
  wks = gsn_open_wks ("x11","poly_ex02")

  res  = True                      ; Plot options desired.

  res@gsnMaximize = True           ; Maximize plot in frame.
  res@gsnFrame    = False          ; Don't advance frame.

  res@trXMinF = min(x)             ; Put limits on X axis.
  res@trXMaxF = max(x)

  plot = gsn_csm_xy (wks,x,y,res)           ; Draw plot, don't advance frame

  mkres               = True       ; Marker options desired
  mkres@gsMarkerIndex = 16         ; Filled circle
  mkres@gsMarkerSizeF = 0.03       ; Make three times larger
  mkres@gsMarkerColor = "Purple"   ; Change color

  gsn_polymarker(wks,plot,-20,-300,mkres)    ; Draw marker at (-20,-300)

  frame(wks)                                 ; Advance the frame.
end