[ENH] groupby_agg() axis argument too restrictive?

[NapsterInBlue]

The Issue

Having some trouble with this line in the current implementation of groupby_agg

In our test suite for this function, we had to get the "count of records in each group" via a complicated numpy call.

We might have wanted to do the same thing with a simpler function call, like so

df.groupby_agg(['date', 'user_id'], 'record_count', 'values', 'count')

But this throws an error

TypeError: count() got an unexpected keyword argument 'axis'

Proposed Change

Taking out the axis=axis portion of the .agg() step, allows for this function call without breaking any of the existing functionality, as far as I (or the unit tests) can tell.

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Can anyone help me find a compelling reason not to make this simple change?