docstr: Helper function, for consistency with the other trends
docstr_tok: ["Helper","function",",","for","consistency","with","the","other","trends"]
code: def _poly_trend(self, trend_coefficients) -> Callable:
      """Helper function, for consistency with the other trends"""
      return np.poly1d(trend_coefficients)
  
imports: from typing import Callable, Optional
  import numpy
  import pandas
  from statsmodels.tsa.stattools import acf
  from darts.logging import get_logger
  from darts.models.forecasting.forecasting_model import LocalForecastingModel
  from darts.timeseries import TimeSeries
  from darts.utils.missing_values import fill_missing_values
label_desc: Use image node_id 4 for calling the FFT obj's underlying member method code with example usage: obj._poly_trend(trend_coefficients) and returns: np