Export vector graphic - transparency

Hi,

I have the following problem:
I want to mark different areas in a plot. therefore I use the fill function. I also want to make those filled areas transparent. This is working fine.

But if I set the FaceAlpha to a value smaller than 1 , the pictures is just a bitmap.
Even when I export it to eps or pdf the pictures looks really bad after zooming a little bit.

example:
figure('PaperUnits','centimeters','PaperPositionMode','manual','Paperposition',[0 0 20 12.3609])
plot(1:0.1:10,sin(1:0.1:10))
print -depsc2 bilder/sinus.eps

figure('PaperUnits','centimeters','PaperPositionMode','manual','Paperposition',[0 0 20 12.3609])
plot(1:0.1:10,sin(1:0.1:10))
hold on
fill([1 1 5 5],[0 0.5 0.5 0],'yellow','FaceAlpha', 0.2,'EdgeAlpha',0)
hold off
print -depsc2 bilder/sinus_fill.eps

in the second file sinus_fill I can't zoom unlimited. the pictures seems to be saved as a bitmap. I also read that this is caused by the renderer (for transparency the OpenGL renderer is used)

Is there any workaround to export transparent plots to a vector image

Thank you
aviak