Is gsn_csm_contour_map_ce overlay impossible?

Dear Lady and Sir,

I want to do an overlay plot with gsn_csm_contour_map_ce. I get the
message that this plot group does not allow overlay. I use 5.1.1. Is
this true or do I do s.th. wrong?
  various statements and setting of workstation
   res = True ; plot mods desired
   res@gsnDraw = False
   res@gsnFrame = False
   res@cnFillOn = True ; turn on color fill
   res@cnFillMode = "RasterFill"
   res@cnLinesOn = False ; turn of contour lines
   res@cnLevelSpacingF = 1. ; contour spacing
   res@gsnSpreadColors = True ; use full range of color map
   res@lbLabelStride = 4
   res@pmTickMarkDisplayMode = "Always"; use NCL default lat/lon labels
   res@gsnAddCyclic = False ; data already has cyclic point
   res@mpMinLatF = 55. ; range to zoom in on
   res@mpMaxLatF = 62.
   res@mpMinLonF =-143.
   res@mpMaxLonF =-127.
   res@mpDataBaseVersion = "HighRes"
   plot1= gsn_csm_contour_map_ce(wks,refhalfdeg({55:62},{-143:-127}), res)
   plot2= gsn_csm_contour_map_ce(wks,scr({55:62},{-143:-127}), res)
   overlay(plot1,plot2)
   draw(plot1)
   frame(wks)
   delete(plot1)
   delete(plot2)
   delete(res)
Thank you very much for letting me know

-- 
Dr. habil. rer. nat. Nicole Mölders, PhD
aka Carmen N. Moelders
Professor of Atmospheric Sciences
Founding chair of the Department of Atmospheric Sciences
Editor-in-Chief of Climate
University of Alaska Fairbanks
Geophysical Institute & College of Natural Science and Mathematics
Department of Atmospheric Sciences
903 Koyukuk Drive
Fairbanks, AK 99775-7320, USA
Phone: +1 907 474 7910
Fax: +1 907 474 7290
URL: http://www.gi.alaska.edu/~molders, http://www.mdpi.com/journal/climate
Email: molders@gi.alaska.edu
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