Could you add a docstring? Currently we are bad with docstrings but it would be nice to improve on this :-)

This is from ggplot()
```
   """
    Statistics, which uses a arbitrary function
    
    longer explanation

    Parameters
    -----------
    x : str
        columnname
    fun :  function
        a function which ...

    Examples
    ----------
    >>> p = ggplot(aes(x='x', y='y'), data=diamonds)
    >>> print(p + geom_point())
    """
```