class_docstr: It is a variation of the AroonUpDown indicator which shows the current
  difference between the AroonUp and AroonDown value, trying to present a
  visualization which indicates which is stronger (greater than 0 -> AroonUp
  and less than 0 -> AroonDown)
  
  Formula:
    - aroonosc = aroonup - aroondown
  
  See:
    - http://stockcharts.com/school/doku.php?id=chart_school:technical_indicators:aroon
code: def _plotinit(self):
      super(AroonOscillator, self)._plotinit()
  
      for yhline in self.plotinfo.plotyhlines[:]:
          self.plotinfo.plotyhlines.append(-yhline)
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  from .None import Indicator, FindFirstIndexHighest, FindFirstIndexLowest
label_desc: Use image node_id 1 for calling the AroonOscillator obj's underlying member method code with example usage: obj._plotinit() without return types