code: def getValue(self):
      ret = None
      if self.windowFull():
          prev = self.getValues()[0]
          actual = self.getValues()[-1]
          ret = utils.get_change_percentage(actual, prev)
      return ret
  
file_path: pyalgotrade/pyalgotrade/technical/ratio.py
imports: from pyalgotrade import technical
  from pyalgotrade import utils
label_desc: Use image node_id 2 for calling the RatioEventWindow obj's underlying member method code with example usage: obj.getValue() and returns: ret