@classmethod
def unpack_scalar(cls, dataset, data):
    """
        Given a dataset object and data in the appropriate format for
        the interface, return a simple scalar.
        """
    if len(data) != 1:
        return data
    key = list(data.keys())[0]
    if len(data[key]) == 1 and key in dataset.vdims:
        return data[key][0]
