def expand(df, column_to_expand):
    """ Returns df with 1 column having multiple values
    expanded to single value cells.

    Parameters:
    ----------
    df: DataFrame
    column_to_expand: str
    """
    colnames = list(df.columns)
    colnames.remove(column_to_expand)
    df.set_index(colnames, inplace=True)
    expanded = df.apply(lambda row: row[column_to_expand], axis=1).stack()
    expanded = expanded.reset_index()
    expanded.columns = colnames + ['year'] + [column_to_expand]
    return expanded