Is it possible to specify a line height/offset amount?

[lukesrw]

I don't seem to be able to find a way of altering the line height, like the yellow circle in PowerPoint:

Which, when dragged further away from the line, increases the total height/offset of the line from the connections.
I would expect if this was possible for there to be an option for a number value in RelationType, but I could be mistaken.

Is this something that the library already supports, if not - would this be easy enough to add?
I'd be happy to take a look and raise a PR if you know where in the code this could be added.