code: def test_stack(self):
      with pytest.raises(ValueError):
          self.series1.stack(self.series3)
      seriesA = self.series1.stack(self.series2)
      dataframeA = pd.concat([self.dataframe1, self.dataframe2], axis=1)
      dataframeA.columns = [
          "0",
          "1",
          "2",
          "0_1",
          "1_1",
          "2_1",
      ]  # the names to expect after stacking
      assert (seriesA.pd_dataframe() == dataframeA).all().all()
      assert seriesA.values().shape == (
          len(self.dataframe1),
          len(self.dataframe1.columns) + len(self.dataframe2.columns),
      )
  
imports: import numpy
  import pandas
  import pytest
  from darts import TimeSeries
  from darts.tests.test_timeseries import TestTimeSeries
label_desc: Use image node_id 14 for calling the TestTimeSeriesMultivariate obj's underlying member method code with example usage: obj.test_stack() without return types