Where is it written (and who decided) that "Peggy" is short for "Margaret"?

And why didn’t that person (or group, as the case may be) not consult with me, prior to making so important a decision?

And for those who say something like, “that’s automatic; it’s a given,” please keep in mind that Margaret Houlihan on M*A*S*H, was never addressed with that “automatic” nickname.

So, what’s the poop? How did “Peggy” become a nickname for “Margaret,” and why?