overlayを使って，色塗りと等値線などの図を重ねることができる．
一連の例が，http://www.ncl.ucar.edu/Applications/overlay.shtmlにある．(2016/07/11)
たとえば以下の要領
plot = gsn_csm_contour_map(wks,temp,res) ; create the temperature plot plot_ov = gsn_csm_contour(wks,uwnd,sres) ; create the U-wind plot overlay(plot,plot_ov) ; overlay the U-wind plot on the temperature plot draw(plot) ; draw the temperature plot (with the U-wind plot overlaid) frame(wks) ; advance the frame