Formatter in a DataGrid?

[handycam]

I created a custom formatter that takes a decimal number and returns a fraction (string). I can use it with some sample text areas, like so:

<comp:FractionFormatter id="makeFraction" myNumber="{myTI.text}"/> 
<mx:TextInput id="myTI"/> 
<mx:TextArea text="Formatted string is {makeFraction.format(myTI.text)}"/>

But if I try to use it in my DataGrid, like so:

<mx:DataGridColumn dataField="{makeFraction.format(@qty)}" width="35" wordWrap="true"/>

The compile fails with "Error: Access of undefined property qty."

Suggestions?

Here's the formatter:

package myComponents
{
    //Import base Formatter class
    import mx.formatters.Formatter

    public class FractionFormatter extends Formatter {

        // Declare the variable to hold the pattern string.
        public var myNumber:String;

        // Constructor
        public function FractionFormatter() {
            // Call base class constructor.
            super();
        }

        // Override format().
        override public function format(value:Object):String         {
            //Validate value - it must be a nonzero length string.
            if(value.length == 0) {    
                    // Return empty string and error message for zero-length string.
                    error="Zero Length String";
                    return ""
            }

            //If the value is valid, format the string.  
            switch (myNumber) {  
                case ".25" :
                    return "1/4";
                    break; 
				case ".5":
					return "1/2";
					break;
				case ".75":
					return "3/4";
					break;
				case "1":
					return "1";
					break;
				case "1.25":
					return "1 1/4";
					break;

                default :
                    // If formatString is not "upper" or "lower", 
                    //     return empty string and set the error message.
                    error="Invalid Format String";
                    return "";
            }
        }
    }
}

[flexy]

Where's @qty coming from?

[handycam]

It's from the XML data (via E4X) in the data grid:

<item ln1="Widgets" minQ=".25" maxQ="2" qty=".25" />