Why SCIFastMountainRenderableSeries cannot render Gradient Color Fill

Hi Sir/Madam,
I have created a Mountain Series with gradient fill. below is part of my code:

-(SCIRenderableSeriesBase *)getTimeSharePlot:(SCIXyDataSeries*)xySeries{
    SCIFastMountainRenderableSeries * rSeries = [SCIFastMountainRenderableSeries new];
    rSeries.dataSeries = xySeries;
    rSeries.strokeStyle = [[SCISolidPenStyle alloc] initWithColor:[UIColor colorWithRed:0 green:0 blue:0 alpha:1] withThickness:1];
    rSeries.areaStyle = [[SCILinearGradientBrushStyle alloc] initWithColorStart:[UIColor colorWithRed:32/255.0 green:56/255.0 blue:124/255.0 alpha:1.0] finish:[UIColor whiteColor] direction:SCILinearGradientDirection_Vertical];

    return rSeries;
}

When i run the test, the chart is just filling with solid color
Would you please help?
BR,
Tommy