Including a function in a chart's row source

I have a chart on a report with a Row Source that includes two values to be pulled from a form as well as one that needs to be returned by calling a function (GetLargestCategory). Assume that the function involved returns a single-quote delimeted string (e.g. 'Category1'). A sample Row Source is shown below. My question is "What is the proper way to include a function call in the Row Source property?" I have done it as shown below and I get no data back in spite of the fact that, if I manually construct the SQL referencing the form controls, and the function call, I get data back.

--- ROW SOURCE BEGINS ---

PARAMETERS [Forms]![Reporting]![Month] IEEEDouble, [Forms]![Reporting]![Line] Text ( 255 );
TRANSFORM Sum(TimeTaken) AS SumOfTime
SELECT FiscalMonth
FROM Table1
WHERE (SystemMonth=[Forms]![Reporting]![Month])
AND (Line=[Forms]![Reporting]![Line])
AND (Category=GetLargestCategory())
GROUP BY FiscalMonth
PIVOT CategorySubType

--- ROW SOURCE ENDS ---