I'm sure that my ass as to mean myself has taken and continues to take many forms across history.
To begin with a typical example:
I need to make sure my ass is on time for work.
Let us note how this differs in effect from something like:
I need to make sure I am on time for work.
Firstly we'll note that the former differs from the latter in that it creates the abstraction of my ass as seperate from I ... furthermore we note, as an effect, the use of third-person via a subject-object relationship where the object, my ass, is related to I/me in such a way as to triangulate a third abstraction which goes unnamed.
A concept sometimes referred to as Distancing Language can be seen used often in political rhetoric. Remember President Bill Clinton's famous wording:
I did not have sexual relations with that woman.
The intention of this wording is to indicate that the speaker is so far removed from "that woman" that the use of her name, or anything less vague than "that woman" at all, would be unnatural for the speaker. Of course, this is a technique of manipulation in speech.
Sometimes, the strategy that opposes Distancing Language is referred to as Inclusive Language. Much more information on these topics can be found by searching obvious channels.
But not all implementation of language to distance is manipulative, in fact, it can sometimes be playful. And that is what we see with an expression such as my ass when used as a replacement for myself.
A common example:
You really saved my ass.
This achieves at least two effects: one being the communication of distance from the situation in which one found oneself, and two being the invocation of intensity. The idea my ass isn't simply me while i'm not particularly consciously aware or interested - in addition, it is me while i'm unable to protect myself as somewhat synonymous to the slang phrase my back (although the two are not interchangable; a friend may have your back but a friend may not have your ass in the same way).
To address your graph and potential relation to African-American Vernacular English: I doubt any correlations uncovered around that idea will be as potent as the rise of post-modern aesthetic during the 1980s and through present day. Self-referential artwork and media have become not only commonplace but expected.
And the phrase in question is a great example of the post-modern phenomenon's manifestation in slang English vernacular. The pull to utilize self-referential effects is so powerful that even my ass feels compelled to go ahead and do so.