Screenshot:

Normal branch, do a few check-ins, then merge branch with trunk. Would expect graph for trunk (white) to stay as a straight line on the left while the branch (pink) behaves as it does. Trunk curving around the right of the branch and then back seems a bit odd and visually confusing.

The thin line represents a merge.  You didn't merge your branch back into
the trunk - you renamed your branch as "trunk" beginning at check-in
<nowiki>[6523fd3854]</nowiki> and then merged the old trunk into your branch (now called "trunk").

That's an interesting way of doing it.  But it should give the same result
as if you had merge the branch into the trunk.  So no harm done.

The graph correctly shows what you did.  The thin merge-lines never stay on
the same "rail" as the thicker ancestry lines.  That is by design.