[BUG] gitk tree view: lines messed up

Hi,

I'm having problems with gitk not displaying the history tree correctly. The 
lines representing branches are messed up, circles for the individual commits 
are missing (screenshot attached).

I tried to delete ~/.config/git/gitk, even tried a completely fresh user 
account without success. The strangest thing: in an almost identical 
installation I cannot reproduce the issue.

Arch Linux
$ pacman -Q git tk tcl                                                                                                                                                         
git 2.9.3-1
tk 8.6.6-1
tcl 8.6.6-1

I'm running out of ideas. There is no error reported from gitk. Is there any 
way to turn on more verbose debugging output? Thanks for your help!

Regards
Raimar