Steffi Graf's real name is
Stefanie as it is of every
Steffi I have ever known. I just can't imagine someone with the legal name of
Steffi. But, oh well,
Leni is one of the most popular names here right now and it is a diminutive like
Steffi. It's just not common at all to use nicknames as first names.
Just because a name is listed here doesn't automatically make it a first name that is also used in the legal sense. More
German examples that I've noticed are
Uschi,
Seppel and
Didi (?). I doubt anyone bears these as whole names as well.