A figure for drawing a link between a relative locatable and its referenced object.
The link is represented by a straight thin line.
FIXME: Some artifacts are visible when the relative locatable object is dragged, because
the clipping region seems not to be updated quickly enough.
A line, which may have length 0 if the related object has no valid reference.
public void paint(java.awt.Graphics2D g)
Paint the figure. This implementation paints a line if the related object has
a valid reference, and it paints nothing if it hasn't. If the length of the
line exceeds a specific threshold, it is drawn with a different color to
highlight that the reference will eventually be broken.