Origin of "hating on"
Trawling USENET, the earliest reference I found was a review posted from a website to rec.music.hip-hop 19th August 1996:
Jay-Z "Can't Knock the Hustle" (Roc-A-Fella)
O-D.U.B.: Man...and I thought "Ain't No N-" was radio-friendly...guess Jay Z one upped himself on this aimed-for-plat-status single. I'm not hating on it either...it flows well...I'd dance to it in a club and Mary J sounds good. It's not a hip hop classic by any means, that's for sure... Take it for what it's worth...
This also matches up with Sam's answer pointing to a track recorded between 1997 and 1999, and pointing out "most early references to this phrase seem to come out of hip-hop and rap culture".
Edit: Found an earlier reference in alt.rap to "hatin on" from 18th July 1996, again about Jay-Z:
Yo... I could bet my life...that blahzay blahzay new album Blah, Blah,
Blah Will be The SHIT!!!! Thats all i gota say...
And for all you NaS haters...Yo... you all just dont know good shit when
it comes out your ass...so fuck you all
And...JZ's lyrics are alright...but i think his beats are wack as hell...
I dont like his style...its bullshit..
Peace
"How you like me now...i go PoW!!!!"Well 2 outta 3 aint too bad, but take another listen to Jay-Z and peep
his ability to flow with the track while effortlessy damn near
speaking to you person to person. But if you your just hatin on his
material rap style, I can unserstand that cause that shit has more
than had its turn.
Peace
Again from rap/hip-hop culture and interesting to see the nearby haters.
It depends on what you regard as the same phrase. For example, Shakespeare's sonnet 149 ends
But, love, hate on, for now I know thy mind,
Those that can see thou lov'st, and I am blind.
This is an intransitive use. The transitive use where X is hating on Y seems to mean X hates Y is more modern (and unnecessary). Here is an example from the letters page of the April 2000 edition of Vibe
Why are people hating on Cash Money? I understand what Shaleena Smith means, but come on, let them experience their five minutes of success. Let them express their happiness. Don't hate the playas, hate the game. - Janae Podgett, Jamaica, NY
There are several uses (with a g) in Raaid Khan's poem Stop Da Hatin from September 2004.
It may be fair to assume that there's just no data prior to 2009, as suggested by Wikipedia:
Originally, Google neglected updating Google Trends on a regular basis. In March 2007, internet bloggers noticed that Google had not added new data since November 2006, and Trends was updated within a week. Google did not update Trends from March until July 30, and only after it was blogged about, again.[2] Google now claims to be "updating the information provided by Google Trends daily; Hot Trends is updated hourly."
— (Google Trends)
Note that it says "Google did not update Trends from March until July 30 [2007][1]", i.e. it's missing slightly less than one and a half years. But as we can't study the inner workings of Google Trends, lack of data still seems more likely than the phrase suddenly springing into existence. A period of 1½ years for it to pick up seems somewhat reasonable.
1: although the phrasing is slightly ambiguous, the source, by its date of publication, suggests 2007
Note: this argument is of course flawed, and further evidence is needed to verify it - please take it as just an hypothesis.
As others have shown, Google Trends is simply inaccurate on this. If you do a Google News search for "hating on" you'll find several entries from before 2009. The earliest I saw was from 2001.
The earliest reference I could find was on Dr. Dre's album The Chronic 2001. From the song Some L.A. Niggaz:
Anybody hatin on us can [do something unpleasant]
According to Wikipedia, the album was recored between 1997 and 1999, so it existed at least at that point. Most early references to this phrase seem to come out of hip-hop and rap culture.
Actually I first heard it as a kid on an NYC subway in 1992 (I would have been about nine.) The phrase is at least as old as that and probably a bit before; it's origin is from Black American English, not rap music itself (the words out of the hooker's mouth in Taxi Driver are a variant.)