class_docstr: Base class which does the calculation of the AroonUp/AroonDown values and
  defines the common parameters.
  
  It uses the class attributes _up and _down (boolean flags) to decide which
  value has to be calculated.
  
  Values are not assigned to lines but rather stored in the "up" and "down"
  instance variables, which can be used by subclasses to for assignment or
  further calculations
code: def _plotlabel(self):
      plabels = [self.p.period]
      return plabels
  
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 _AroonBase obj's underlying member method code with example usage: obj._plotlabel() and returns: plabels