Market price for 2 oil products

[wsandy59]

I am not a programmer but I'm trying to build a simple indicator that tells me the difference between 2 products. I have entered coding as shown below & compiled without errors. Unfortunately the output doesn't appear on a chart. Can someone provide a tip. 

//--- Seek prices for XBRUSD and XTIUSD

              
              double vbid   = MarketInfo("XBRUSD",MODE_BID);
              double vask   = MarketInfo("XTIUSD",MODE_ASK); 
              double vspread  = vbid - vask;
              //--- Output values on screen
   Comment(StringFormat("Spread",vspread));