Convert dimension values to DataFrame.

        Returns a pandas dataframe of columns along each dimension,
        either completely flat or indexed by key dimensions.

        Args:
            dimensions: Dimensions to return as columns
            multi_index: Convert key dimensions to (multi-)index

        Returns:
            DataFrame of columns corresponding to each dimension

def dframe(self, dimensions=None, multi_index=False):
        """Convert dimension values to DataFrame.

        Returns a pandas dataframe of columns along each dimension,
        either completely flat or indexed by key dimensions.

        Args:
            dimensions: Dimensions to return as columns
            multi_index: Convert key dimensions to (multi-)index

        Returns:
            DataFrame of columns corresponding to each dimension
        """
        if dimensions:
            dimensions = [self.get_dimension(d, strict=True) for d in dimensions]
        else:
            dimensions = self.kdims
        vdims = [d for d in dimensions if d in self.vdims]
        if vdims:
            raise ValueError('%s element does not hold data for value '
                             'dimensions. Could not return data for %s '
                             'dimension(s).' %
                             (type(self).__name__, ', '.join([d.name for d in vdims])))
        return super(StatisticsElement, self).dframe(dimensions, False)