You may follow this example:

Table::create(array(
    ...
    "grouping"=>array(
        "year"=>array(
            "calculate"=>array(
                "{sumAmount}"=>array("sum","amount"),
                "{avgAmount}"=>array("avg","amount")
                "{maxAmount}"=>array("max","amount")
            ),
            "top"=>function($calculated_results){
                return "Number items:".$calculated_results["count"];
            },
            "bottom"=>function($calculated_results){
                return "Sum amount:".$calculated_results["{sumAmount}"];
            },
        ),
    ),
));

You may see in "top" and "bottom" anonymous function, you receive calculated results in an array, so basically you may use the number_format() function to format them ( to add decimals that you want) before return the string for group top and group bottom.