percentage value precision

[Tom Fitzpatrick]

I'm using a display function to show percentage values in a pie chart 
(thanks to the earlier suggestion from this list), as follows:

function display(data, field, index, percentValue) {
return data.Expense + ": " + percentValue + "%";
}
Now what I'd like to do is have the percentValue returned with a precision 
of only two points past the decimal. I can set this up with a 
NumberFormatter, but what is the syntax for putting it into action?