code: def test_boxcox_multi_ts(self):
      test_cases = [
          ([[0.2, 0.4], [0.3, 0.6]]),  # full lambda
          (0.4),  # single value
          None,  # None
      ]
  
      for lmbda in test_cases:
          box_cox = BoxCox(lmbda=lmbda)
          transformed = box_cox.fit_transform(
              [self.multi_series, self.multi_series]
          )
          back = box_cox.inverse_transform(transformed)
          pd.testing.assert_frame_equal(
              self.multi_series.pd_dataframe(),
              back[0].pd_dataframe(),
              check_exact=False,
          )
          pd.testing.assert_frame_equal(
              self.multi_series.pd_dataframe(),
              back[1].pd_dataframe(),
              check_exact=False,
          )
  
file_path: darts/darts/tests/dataprocessing/transformers/test_boxcox.py
imports: from copy import deepcopy
  import numpy
  import pandas
  import pytest
  from darts import TimeSeries
  from darts.dataprocessing.transformers import BoxCox, Mapper
  from darts.utils.timeseries_generation import linear_timeseries, sine_timeseries
label_desc: Use image node_id 4 for calling the TestBoxCox obj's underlying member method code with example usage: obj.test_boxcox_multi_ts() without return types