I think it's the descending node line as it curves down and is marked Ds at the bottom. I would be grateful to know the significance.
Incidentally, I was taken to these places by accident as my company posted me here and not my personal desire. I felt I had a strong affinity with poland/ russia ( past life!?), but couldn't resonate with germany. Italy was ok.
the line continues down thru the south of france ( appealing), and down through morocco ( much less appealing).
Thanks also for the link on Mercury. It's a convincer for me as the previous two retrograde cycles had similar activity in terms of bookings, cancellations etc.