def get_primary_data(data):
    """
    Get the anchor of multiple data series according to data length

    Args:
        data(dict): e.g. {
            "data1": [[1,2]],
            "data2": [[2,3], [4,5]]
        }

    Returns:
        str: key of the anchor data
    """
    res = sorted(data, key = lambda k: len(data[k]))
    return res[0]