code: def get_change_percentage(actual, prev):
      if actual is None or prev is None or prev == 0:
          raise Exception("Invalid values")
  
      diff = actual - prev
      ret = diff / float(abs(prev))
      return ret
  
label_desc: Use image node_id 1 for calling a global function with example usage: get_change_percentage(actual, prev) and returns: ret