class_docstr: Defined by John Bollinger in the 80s. It measures volatility by defining
  upper and lower bands at distance x standard deviations
  
  Formula:
    - midband = SimpleMovingAverage(close, period)
    - topband = midband + devfactor * StandardDeviation(data, period)
    - botband = midband - devfactor * StandardDeviation(data, period)
  
  See:
    - http://en.wikipedia.org/wiki/Bollinger_Bands
code: def _plotlabel(self):
      plabels = [self.p.period, self.p.devfactor]
      plabels += [self.p.movav] * self.p.notdefault("movav")
      return plabels
  
file_path: backtrader/backtrader/indicators/bollinger.py
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  from .None import Indicator, MovAv, StdDev
label_desc: Use image node_id 1 for calling the BollingerBands obj's underlying member method code with example usage: obj._plotlabel() and returns: plabels