Added a strokeWidth to lines. Circles for points adapt as well.

This pull request adds a strokeWidth to lines. It also adds the option to treat this strokeWidth as device independent pixels. This way lines look the same no matter which device. The defaults (6 for strokeWidth and false for treating the width as dips) are set so that existing graphs stay the same.

Since the current point markers wouldn't look too well for lines with a different strokeWidth the radius also takes the strokeWidth into account.