Callback function to allow for dynamic tooltip generation

It should be possible to do:
content : function(xval, yval){
   if(xval==1234){
       return '<div>'+yval*2+'</div>';
   } else {
       return '<div>'+yval*yval+'</div>';
   }
}

I tried to keep the code in the same format you have. If you accept the pool I suggest you to review the documentation I pushed. Hope it helps ;)