Graduate student in or of?

Solution 1:

I would say in Statistics.

The reason I say this is because you would say: I am majoring in Statistics.

Or in response to the question: What do you do for a living? You might say, "I'm in plastics."

You can say I'm a student of Statistics, and hence, it wouldn't be wrong to say graduate student of Statistics. But, I don't believe it is the common usage (in American English).

Solution 2:

In Britain we do not 'major' in things.

As undergraduates we 'read' subjects. Even a chemist, spending 75% of their time in a laboratory, would be 'reading' chemistry.

At least that was always the case but younger people may well tell me I'm wrong.

For that reason I would say 'I am a post-graduate student of statistics'.