Grade line is not centred

[Jörg Kienzle]

When a grade is associated to a model element, e.g., a class, there is a dotted line drawn between the MarkObject and the Class.

The line should be connected to the center of the circle. However, when one zooms in, it looks like this:

And when one zooms out, it looks like this:

Probably the starting point for the line is calculated absolutely instead of relatively.

[Geoffrey Taerim Kim]

Fix PR submitted