code: def test_no_grouper(self, df):
      groupby = GroupBy(["group"])
      res = PolyFit(order=1, gridsize=100)(
          df[["x", "y"]], groupby, "x", {}
      )
  
      assert_array_equal(res.columns, ["x", "y"])
  
      grid = np.linspace(df["x"].min(), df["x"].max(), 100)
      assert_array_equal(res["x"], grid)
      assert_array_almost_equal(
          res["y"].diff().diff().dropna(), np.zeros(grid.size - 2)
      )
  
file_path: seaborn/tests/_stats/test_regression.py
imports: import numpy
  import pandas
  import pytest
  from numpy.testing import assert_array_equal, assert_array_almost_equal
  from pandas.testing import assert_frame_equal
  from seaborn._core.groupby import GroupBy
  from seaborn._stats.regression import PolyFit
label_desc: Use image node_id 2 for calling the TestPolyFit obj's underlying member method code with example usage: obj.test_no_grouper(df) without return types