code: def average_true_range(close_data, period):
      
      tr = true_range(close_data, period)
      atr = smoothed_moving_average(tr, period)
      atr[0:period-1] = tr[0:period-1]
      return atr
docs: Average True Range.
  
      Formula:
      ATRt = ATRt-1 * (n - 1) + TRt / n