I've played Ahri a crap ton since I started at the beginning of S3, If champ mastery started when I started I'd prolly be close to a mil on her.
That being said, that spike you see was a buff to her Orb, .5 AP Ratio addition both ways (so .10 if you hit both, raising it from .60 AP scaling to .70 AP scaling both ways combined) and a mana buff to her Q.
Most people didn't like her during her Season 4 kiss of death kit, it essentially meant if you hit the charm, they died, if you missed you didn't do much damage.
The reason I remember this is cause I played her a lot, specially back then, When DFG was removed, they removed her kiss of death and added the MS on the Orb, plus buffed her Charms base and AP ratio to compensate.
A lot of the reason people complain about her. now more so then before is because the semi bad match ups she had, have been nerfed (LB/Zed/Syndra for example), she's maintained a good win rate, but that's imo is more so because she is a skill match up champ, with a lot of safety, and the people who play her are the people who for the most part have played her a lot, she's a popular champion with a big fan base.
If they want to fix Ahri without breaking her, they should just remove the MS on her Orb and add the Damage Amp to her Charm, while reducing the Charm's damage. Essentially putting her back in Season 4 status, but now that other champs are on a similar level as she would of been back then, she should even back out.
But I'll prolly get downvoted for this soooo.