text: Title: ValueError: Field "duration" has type "timedelta64[ns]" which is not supported by Altair.
  Body: It worked fine with 2 columns of datetimes and 2 columns of integers. I used df.apply(f(x)) to create a timedelta column and got the following warning. Full text:
  
  
  /opt/conda/lib/python3.7/site-packages/IPython/core/formatters.py:918: UserWarning:
  Unexpected error in rendering Lux widget and recommendations. Falling back to Pandas display.
  Please report the following issue on Github: https://github.com/lux-org/lux/issues 
  
  /opt/conda/lib/python3.7/site-packages/lux/core/frame.py:632: UserWarning:Traceback (most recent call last):
    File "/opt/conda/lib/python3.7/site-packages/lux/core/frame.py", line 594, in _ipython_display_
      self.maintain_recs()
    File "/opt/conda/lib/python3.7/site-packages/lux/core/frame.py", line 451, in maintain_recs
      self._widget = rec_df.render_widget()
    File "/opt/conda/lib/python3.7/site-packages/lux/core/frame.py", line 681, in render_widget
      widgetJSON = self.to_JSON(self._rec_info, input_current_vis=input_current_vis)
    File "/opt/conda/lib/python3.7/site-packages/lux/core/frame.py", line 721, in to_JSON
      recCollection = LuxDataFrame.rec_to_JSON(rec_infolist)
    File "/opt/conda/lib/python3.7/site-packages/lux/core/frame.py", line 749, in rec_to_JSON
      chart = vis.to_code(language=lux.config.plotting_backend, prettyOutput=False)
    File "/opt/conda/lib/python3.7/site-packages/lux/vis/Vis.py", line 334, in to_code
      return self.to_vegalite(**kwargs)
    File "/opt/conda/lib/python3.7/site-packages/lux/vis/Vis.py", line 310, in to_vegalite
      self._code = renderer.create_vis(self)
    File "/opt/conda/lib/python3.7/site-packages/lux/vislib/altair/AltairRenderer.py", line 99, in create_vis
      chart_dict = chart.chart.to_dict()
    File "/opt/conda/lib/python3.7/site-packages/altair/vegalite/v4/api.py", line 373, in to_dict
      dct = super(TopLevelMixin, copy).to_dict(*args, **kwargs)
    File "/opt/conda/lib/python3.7/site-packages/altair/utils/schemapi.py", line 328, in to_dict
      context=context,
    File "/opt/conda/lib/python3.7/site-packages/altair/utils/schemapi.py", line 62, in _todict
      for k, v in obj.items()
    File "/opt/conda/lib/python3.7/site-packages/altair/utils/schemapi.py", line 63, in <dictcomp>
      if v is not Undefined
    File "/opt/conda/lib/python3.7/site-packages/altair/utils/schemapi.py", line 58, in _todict
      return [_todict(v, validate, context) for v in obj]
    File "/opt/conda/lib/python3.7/site-packages/altair/utils/schemapi.py", line 58, in <listcomp>
      return [_todict(v, validate, context) for v in obj]
    File "/opt/conda/lib/python3.7/site-packages/altair/utils/schemapi.py", line 56, in _todict
      return obj.to_dict(validate=validate, context=context)
    File "/opt/conda/lib/python3.7/site-packages/altair/vegalite/v4/api.py", line 363, in to_dict
      copy.data = _prepare_data(original_data, context)
    File "/opt/conda/lib/python3.7/site-packages/altair/vegalite/v4/api.py", line 84, in _prepare_data
      data = _pipe(data, data_transformers.get())
    File "/opt/conda/lib/python3.7/site-packages/toolz/functoolz.py", line 627, in pipe
      data = func(data)
    File "/opt/conda/lib/python3.7/site-packages/toolz/functoolz.py", line 303, in __call__
      return self._partial(*args, **kwargs)
    File "/opt/conda/lib/python3.7/site-packages/altair/vegalite/data.py", line 19, in default_data_transformer
      return curried.pipe(data, limit_rows(max_rows=max_rows), to_values)
    File "/opt/conda/lib/python3.7/site-packages/toolz/functoolz.py", line 627, in pipe
      data = func(data)
    File "/opt/conda/lib/python3.7/site-packages/toolz/functoolz.py", line 303, in __call__
      return self._partial(*args, **kwargs)
    File "/opt/conda/lib/python3.7/site-packages/altair/utils/data.py", line 149, in to_values
      data = sanitize_dataframe(data)
    File "/opt/conda/lib/python3.7/site-packages/altair/utils/core.py", line 317, in sanitize_dataframe
      "".format(col_name=col_name, dtype=dtype)
  ValueError: Field "duration" has type "timedelta64[ns]" which is not supported by Altair. Please convert to either a timestamp or a numerical value.