[ncl-talk] Overlaying two plots onto the same contour map

[G.Monte at isac.cnr.it]

Dears all,
I have some problems in overlaying two plots onto the same contour map.
The data fields to be overlayed are arranged into an ascii file and are
read in the following way by NCL 6.4.0:
;---read longitude
lon  = dataf(:,4)

;---read latitude
lat  = dataf(:,3)

;---read tb
tb   = dataf(:,6)

;---read data
flag = dataf(:,11)

For plotting them, I follwed the template shown in overlay_12.ncl, where I
set:
;---Create the two plots
  tb_plot = gsn_csm_contour_map(wks,tb,tres)
  vv_plot = gsn_csm_contour(wks,flag,dres)
;---Overlay the vv plot on the tb plot
  overlay(tb_plot,vv_plot)
;---Drawing the tb plot will also draw vv plot
  draw(tb_plot)
  frame(wks)

By doing so, only the second data field (named "flag") appeared in the
plot, while the first field (named "tb") did not; it appears only if I
plot them separately.
I really don't understand the reason, I kindly ask you a suggestion.

Thank you,
Giulio