class_docstr: Subclass of MACD which adds a "histogram" of the difference between the
  macd and signal lines
  
  Formula:
    - histo = macd - signal
  
  See:
    - http://en.wikipedia.org/wiki/MACD
code: def __init__(self):
      super(MACDHisto, self).__init__()
      self.lines.histo = self.lines.macd - self.lines.signal
  
file_path: backtrader/backtrader/indicators/macd.py
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  from .None import Indicator, MovAv
label_desc: Use image node_id 1 to create a new MACDHisto object from inherited base classes: MACD with example: obj = MACDHisto()