Such a thing will often happen as the result of a celebrity having the name. For instance, famed Fleetwood Mac singer Stevie Nicks, whose real name was Stephanie.

With the shift happening right after the 1976–1980 square, I'd assume people named their daughter after Stevie Nicks.

In the US, boys would abbreviate Stephen or Steven to Steve. Stevie only for a young boy.

I’m not sure what they are measuring here though - if it’s only birth certificates then I’m sure Stevie is dominated by girls for a long time.

I mean, this is US-centric data, but it is probably a shortening of 'Stephanie' or 'Stefanie' [1], the female variant of 'Stephan' or 'Stefan'. It stems from Greek, apparently: Stephanos. Its variant are used entirely throughout Europe, from Portugal to Russia.

[1] https://en.wikipedia.org/wiki/Stephanie

Now? Let me introduce you to Stevie Nicks.

Don't be silly.

Moon Unit is a girls name.

Stevie is a nickname for the sweetener Stevia.