[amibroker] plot total volume for watchlist

[CoolAussieGuy]

Hi I am trying to plot total volume for symbols in a watchlist.
The code below works but when a symbol in the watchlist has no data for a date 
the total volume is always returned as 0 for the whole watchlist. Please help, 
Thanks :)


listnum = Param("Watchlist Number",0,0,100,1);
function GetVolumeForWatchList( listnum ) 
{ 
   // retrive comma-separated list of symbols in watch list 
   list = CategoryGetSymbols( categoryWatchlist, listnum );    
   Vol = 0;

   for( i = 0; ( sym = StrExtract( list, i ) ) != ""; i++ ) 
   { 
      f = Foreign( sym, "V" ); 
      Vol = Vol + f;    
      _TRACE("Vol= " + Vol);
   } 
        
   return Vol;
} 
Plot(GetVolumeForWatchList(listnum), "Total Volume", colorGreen, styleHistogram 
);