class_docstr: MixIn class to create a subclass with another indicator. The main line of
  that indicator will be surrounded by an upper and lower band separated a
  given "perc"entage from the input main line
  
  The usage is:
  
    - Class XXXEnvelope(XXX, EnvelopeMixIn)
  
  Formula:
    - 'line' (inherited from XXX))
    - top = 'line' * (1 + perc)
    - bot = 'line' * (1 - perc)
  
  See also:
    - http://stockcharts.com/school/doku.php?id=chart_school:technical_indicators:moving_average_envelopes
code: def __init__(self):
      # Mix-in & directly from object -> does not necessarily need super
      # super(EnvelopeMixIn, self).__init__()
      perc = self.p.perc / 100.0
  
      self.lines.top = self.lines[0] * (1.0 + perc)
      self.lines.bot = self.lines[0] * (1.0 - perc)
  
      super(EnvelopeMixIn, self).__init__()
  
file_path: backtrader/backtrader/indicators/envelope.py
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import sys
  from .None import Indicator, MovingAverage
label_desc: Use image node_id 1 to create a new EnvelopeMixIn object from inherited base classes: object with example: obj = EnvelopeMixIn()