Yeah a daisy isn't a marguerite but they come from the same family of flowers and sometimes a marguerite is called a daisy margurite in English so I think it's ok even though I wouldn't do it.
I think it could possibly come from Margaret-->Maisie-->Daisy. It has been done for a long time, wasn't the daughter of one of the Little Women called
Margaret nn
Daisy? I guess it happened at a time when many many people were named
Margaret and you had to make up nicknames in order to tell them all apart.
Margaret nn
Peggy also doesn't make sense. Neither do
William nn
Bill or
Mary nn
Mamie.
I found an interesting article about nicknames:
http://www.namenerds.com/uucn/advice/nickhistory.html