GradientPen direction

Hi!

I'm trying to create a ribbon effect using the DrawArc that goes from solid colour to transparent. It appears that the GradientPen creates gradients along the width of the shape, but I'd like it to gradient along the length of the shape instead.

Is this possible? I tried changing line 52 in the GradientPen to use lengthPosition instead, but then I just get a solid colour.

Thanks!