Calculate the Bolinger indicator value

    >>> symbol_bollinger("goog", '2008-1-1', '2008-2-1')[-1]  # doctest: +ELLIPSIS, +NORMALIZE_WHITESPACE
    -1.8782...

def symbol_bollinger(symbol='GOOG',
    start=datetime.datetime(2008, 1, 1), end=datetime.datetime(2009, 12, 31), price_type='close', cleaner=clean_dataframe,
    window=20, sigma=1.):
    """Calculate the Bolinger indicator value

    >>> symbol_bollinger("goog", '2008-1-1', '2008-2-1')[-1]  # doctest: +ELLIPSIS, +NORMALIZE_WHITESPACE
    -1.8782...
    """
    symbols = normalize_symbols(symbol)
    prices = price_dataframe(symbols, start=start, end=end, price_type=price_type, cleaner=cleaner)
    return series_bollinger(prices[symbols[0]], window=window, sigma=sigma, plot=False)