That is really really upsetting.
Out of interest (and please don't take this the wrong way - I know nothing about the plane or whatever), is there a checklist or something where you could tell during pre-flight checks whether it was track up or true north.
I'm just asking as maybe it was legitamately set to the other before your flight. The fact that you didn't know about it could be worth noting in a new edition of a POH where it explicitly mentions to check this instrument.