How to create vertical gradient in flex.

What i currently have is a horizontal color gradient. And its working fine. But I am unable to figure out how should I make it vertical (which ofcourse is the requirement)

I use styleName = "chatWindowLeftGradient"

<mx:VBox id="chatTabBarVBox" height="100%" styleName="chatWindowLeftGradient">

</mx:VBox> 

And the style sheet looks like this:

<mx:Style>
        .chatWindowLeftGradient{                        
            backgroundImage: ClassReference("custom.GradientBackground");
            backgroundSize: "100%";
            fillColors: #6db263, #a4d9a1;
            fillAlphas: 1, 1;                           
        }

    </mx:Style>

This give a gradient from top to bottom. How i can make from left to right??

Regards Zeeshan