Unless I am, I think, missing something completely, this is a classic case where the Ngram is the wrong tool for the job.
"I am myself" sounds rather natural, while "I am I" sounds oddly unfamiliar. As such, I'd expect "I am myself" to completely trounce "I am I", instead of being relatively close like the Ngram shows:
But this is where the data ought to be investigated further. By using the links on the bottom, one can see that "I am myself" is found in several works, in a very natural-sounding context:
Whereas "I am I", more often than not, is the result of a pure coincidence, such as when one sentence or clause ends with "I am", and the next begins with "I":
To the O.P.: everything depends on context. That said, most of the time, when you are referring to yourself, you would probably say "I am myself," and not "I am I."
And be very careful with how you use Ngrams. I am myself very leery about looking at the lines, and leaving it at that.