def METHOD_NAME(self, agg_dict):
    """Returns an aggregation object from an aggregation group dict.
    Args:
        agg_dict (dict): a dictionary containing information
            about the stored aggregation group.
    Returns:
        A dict that contains metadata about the aggregation group
        as well as a chart object (instance of altair.Chart)
        with the combined chart object from the group.
    """
    raise NotImplementedError