unknown diagonal white line on gsn_csm_map plot

Hi,

why is there a diagonal white line from the lower left corner to upper right?
How can I prevent this?

ncl-script:

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

begin

  wks_type = "png"
  wks_name = "latlines"
  wks = gsn_open_wks(wks_type, wks_name) ; Open a workstation.

  res = True
  res@gsnFrame = False ; Don't draw or advance
  res@gsnDraw = False ; frame yet.
  res@gsnMaximize = True

;---This will be our base, so make it a map plot.
  ;res@tfDoNDCOverlay = False
  ;res@gsnAddCyclic = False
  res@mpProjection = "CylindricalEquidistant" ; Projection
  res@mpOutlineOn = True
  res@mpDataBaseVersion = "MediumRes"
  res@mpOutlineBoundarySets = "AllBoundaries"
  ;res@mpOutlineBoundarySets = "National" ; country borders
  res@mpNationalLineThicknessF = 2.0
  res@mpLimitMode ="LatLon"
  res@mpMinLatF = -21.87
  res@mpMaxLatF = -11.09
  res@mpMinLonF = 14.21
  res@mpMaxLonF = 25.87

  Map = gsn_csm_map(wks, res)

  pres = True ; polyline mods desired
  pres@gsLineThicknessF = 2.0 ; line thickness
  pres@gsLineColor = "blue" ; color of lines
  
  val = gsn_add_polyline(wks,Map,(/14.21,25.87/),(/-12.63,-12.63/),pres)

  draw(Map)
  frame(wks)

  ;delete(res)
  
  ;make high density png with imagemagick
  ;delete(wks)

  options = "mogrify -strip -trim -geometry 1000x1000 -density 300 -units pixelsperinch " + wks_name + ".png"
  ;system(options)

end
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Received on Tue Dec 18 12:44:03 2012