def stack_df(df, index_='Datetime'):
    """
            Stack dataframe df with index index_
            :param df: dataframe with index_ index and columns corresponding to
            stop id
            :param index_: str index name of dataframe 
    """
    df_ = df.copy()
    df_ = df_.set_index('Datetime').stack().reset_index()
    df_.columns = [index_, 'stop_id', 'value']
    return df_