var1 = strmid(cmd.var, 0, idx)
               fldr1 = nc_read(file_name, var1, ncdf_db, BOXZOOM = box_plot, $
                               TIME_1 = time1,  TIME_2 =  time2, ALL_DATA = all_data, _extra = ex)
               file_name1 = file_name
               ncdf_db1 = ncdf_db
               CASE cmd.grid OF 
                  'U': vargrid1 = 'U'
                  'V': vargrid1 = 'V'
                  ELSE: vargrid1 = 'T'
               ENDCASE 
               stddev_mth =  '00'
               ; read second variable
               IF strpos(cmd.var, '(next)') EQ -1 THEN BEGIN  ; type 1 y=f(x) on 1 line
                  var2 = strmid(cmd.var, idx+3, strlen(cmd.var)-idx-4)
                  ; check months to analyse
                  pos_underscore =  strpos(var2,'_')
                  IF pos_underscore GE 0 THEN BEGIN
                     stddev_mth = STRMID(var2, pos_underscore + 1, 6)
                     var2 = STRMID(var2, 0, pos_underscore)
                  ENDIF
                  fldr2 = nc_read(file_name, var2, ncdf_db, BOXZOOM = box_plot, $
                                  TIME_1 = time1,  TIME_2 =  time2, ALL_DATA = all_data, _extra = ex)
                  datyp2 = datyp
                  cmd2 = cmd
                  sw_diffg = 0
                  vargrid2 = vargrid1
               ENDIF ELSE  BEGIN  ; type 2 y=f(next) or y=bin(x) on 2 lines
                  cmd2 = decode_cmd(cmdline_main, idx_main+1)
                  ; check months to analyse
                  pos_underscore =  strpos(cmd2.var,'_')
                  IF pos_underscore GE 0 THEN BEGIN
                     stddev_mth = STRMID(cmd2.var, pos_underscore + 1, 6)
                     cmd2.var = STRMID(cmd2.var, 0, pos_underscore)
                  ENDIF
                  cmd2.timave = cmd.timave
                  cmd2.date1 = cmd.date1
                  cmd2.spec = cmd.spec
                  datyp2 = def_dptyp(cmd2)
                  cmd2_back = cmd2
                  ; other grid ?
                  grid2_test = cmd2.grid
                  sw_diffg = 0
                  IF grid2_test NE cmd.grid THEN BEGIN 
                     sw_diffg = 1
                     jptb =  jpt
                     IF debug_w THEN print, '    (in data_read) cmd2.grid <> cmd.grid ', cmd2.grid, cmd.grid
                     IF debug_w THEN print, '    exec def_grid, cmd2_back in data_read : '
                     grid2_full = cmd2_back.grid
                     def_grid, cmd2_back
                     jpt = jptb
                  ENDIF 

                  fldr2 = data_read(cmd2, datyp2.hotyp, datyp2.plttyp, '1', '0', ALL_DATA = all_data, _extra = ex)
                  var2 = cmd2.var
                  print,  '   Variable 2 ',  var2,  ' read on vargrid ',  vargrid
                  jptb =  jpt
                  if debug_w THEN print, '   yfx grids different ? / sw_diffg = ', sw_diffg
                  IF sw_diffg EQ 1 THEN BEGIN
                     IF debug_w THEN print, '   exec def_grid, cmd1_back in data_read : '
                     grid1_full = cmd1_back.grid
                     def_grid, cmd1_back
                     IF read_grid_from_file EQ 1 THEN BEGIN 
                        mesh_from_file, cmd1_back.grid, file_name1, ncdf_db1, cmd1_back.var
                        key_shift_map = key_shift
                     ENDIF 
                  ENDIF
                  jpt = jptb
                  CASE cmd2.grid OF 
                     'U': vargrid2 = 'U'
                     'V': vargrid2 = 'V'
                     ELSE: vargrid2 = 'T'
                  ENDCASE
               ENDELSE  
               ; consistency check
               IF fldr1.dim NE fldr2.dim THEN BEGIN
                  print, '  *** dimension inconsistency in y=f(x) : ', cmd.var
                  return, -1
               ENDIF