DKRZ PyNGL example overlay

Example:

from __future__ import print_function
import numpy as np
import Ngl,Nio

#-----------------------------------------------------------------------
#-- Function: main
#-----------------------------------------------------------------------
def main():

    #--  open file and read variables
    f    = Nio.open_file("rectilinear_grid_3D.nc", "r")
    t    = f.variables["t"][0,0,:,:]
    rhum = f.variables["rhumidity"][0,0,:,:]
    lat  = f.variables["lat"][:]
    lon  = f.variables["lon"][:]

    #-- define workstation
    wks =  Ngl.open_wks("png","plot_overlay")

    #-- plot resources for the map
    mpres                =  Ngl.Resources()
    mpres.nglDraw        =  False              #-- don't draw plot
    mpres.nglFrame       =  False              #-- don't advance frame
    mpres.mpOutlineOn    =  True               #-- turn on map outlines
    mpres.mpGeophysicalLineColor = "gray50"    #-- map outline color
    mpres.mpLimitMode    = "LatLon"            #-- limit map via lat/lon
    mpres.mpMinLatF      =  20.0               #-- min lat
    mpres.mpMaxLatF      =  60.0               #-- max lat
    mpres.mpMinLonF      = -10.0               #-- min lon
    mpres.mpMaxLonF      =  40.0               #-- max lon

    #-- plot resources for the temperature plot
    tres                 =  Ngl.Resources()
    tres.nglDraw         =  False              #-- don't draw plot
    tres.nglFrame        =  False              #-- don't advance frame
    tres.cnFillOn        =  True               #-- turn on color fill
    tres.cnFillPalette   = "cmp_b2r"           #-- set the colormap to be used
    tres.cnLinesOn       =  False              #-- turns off contour line labels
    tres.cnLineLabelsOn  =  False              #-- turns off contour line labels
    tres.cnInfoLabelOn   =  False              #-- turns off contour info label

    tres.cnLevelSelectionMode = "ManualLevels" #-- select manual levels
    tres.cnMinLevelValF  =  240.               #-- minimum contour value
    tres.cnMaxLevelValF  =  310.               #-- maximum contour value
    tres.cnLevelSpacingF =  2.                 #-- contour increment

    tres.pmLabelBarOrthogonalPosF = -0.26 #-- move labelbar upward
    tres.lbLabelFontHeightF =  0.009           #-- labelbar labe font size
    tres.lbBoxMinorExtentF  =  0.17            #-- decrease height of labelbar box
    tres.lbOrientation   = "horizontal"        #-- horizontal labelbar

    tres.tiMainString    = "Colors: temperature, lines: rhumitity"
                                               #-- title string
    tres.sfXArray        =  lon
    tres.sfYArray        =  lat

    #-- plot resources for the rel. humidity plot
    rres                 =  Ngl.Resources()
    rres.nglDraw         =  False              #-- don't draw plot
    rres.nglFrame        =  False              #-- don't advance frame

    rres.cnInfoLabelOrthogonalPosF = 0.13      #-- move info label upward

    rres.sfXArray        =  lon
    rres.sfYArray        =  lat

    #-- generate the map base plot, but don't draw it yet
    map   = Ngl.map(wks,mpres)

    #-- generate temperature plot, but don't draw it yet
    tplot = Ngl.contour(wks,t,tres)

    #-- generate rel. humidity plot, but don't draw it yet
    rplot = Ngl.contour(wks,rhum,rres)

    #-- overlay rplot on tplot
    Ngl.overlay(map, tplot)
    Ngl.overlay(map, rplot)

    #-- draw the plot
    Ngl.draw(map)

    #-- add annotation of variable long_name and units to the plot
    txres               = Ngl.Resources()
    txres.txFontHeightF = 0.014

    Ngl.text_ndc(wks,f.variables["t"].attributes['long_name'],0.17,0.88,txres)
    Ngl.text_ndc(wks,f.variables["t"].attributes['units'],    0.95,0.88,txres)

    #-- advance the frame
    Ngl.frame(wks)

if __name__ == '__main__':
    main()

Result: