@classmethod
def compute_group(cls, data, scales, **params):
    """
        Calculate statistics for the group

        All stats should implement this method

        Parameters
        ----------
        data : dataframe
            Data for a group
        scales : types.SimpleNamespace
            x (``scales.x``) and y (``scales.y``) scale objects.
            The most likely reason to use scale information is
            to find out the physical size of a scale. e.g::

                range_x = scales.x.dimension()

        params : dict
            Parameters
        """
    msg = '{} should implement this method.'
    raise NotImplementedError(msg.format(cls.__name__))
