code: def _roll_data(self):
          
  
          self.buffer.values[:, :self._window, :] = \
              self.buffer.values[:, -self._window:, :]
          self.date_buf[:self._window] = self.date_buf[-self._window:]
          self._pos = self._window
docs: Roll window worth of data up to position zero.
          Save the effort of having to expensively roll at each iteration