code: def _generate_polynomial_series(n, order, coefs=None):
      """Generate polynomial series of given order and coefficients."""
      if coefs is None:
          coefs = np.ones((order + 1, 1))
      x = np.vander(np.arange(n), N=order + 1).dot(coefs)
      return x.ravel()
  
file_path: sktime/sktime/utils/_testing/forecasting.py
imports: import numpy
  import pandas
  from sktime.forecasting.base import ForecastingHorizon
  from sktime.utils._testing.series import _make_series
  from sktime.utils.validation.forecasting import check_fh
label_desc: Use image node_id 3 for calling a global function with example usage: _generate_polynomial_series(n, order, coefs) and returns: x