Silverlight is not a proper answer to drawing charts on an Windows Embedded Compact device. You should go for using (or creating) a native GDI element. If performance is very important (e.g. for a fast-updating chart), you might also consider DirectDraw.
The best way in a commercial project would probably be to use a readymade component, such as this one.
You can find somebody else's experiences and advice here for pointers to what to look for when doing this yourself. If you expand your post with more details about what you are actually trying to achieve, I might give you a more detailed response.