`datashader=True` caused `subplots=True` to raised `KeyError: 'Variable`

Bug mentioned in https://github.com/holoviz/hvplot/issues/486 re-emerged. Whenever `datashade=True` or `rasterize=True`, `KeyError: 'Variable'` will be raised.

Environments:
datashader==0.14.4
holoviews==1.15.4
hvplot==0.8.3
panel==0.14.4

This bug can be reproduced using example given in this thread or

```python
from hvplot.sample_data import airline_flights, us_crime
us_crime.hvplot(x='Year', y=['Burglary rate', 'Violent Crime rate', 'Robbery rate'],
value_label='Rate', subplots=True, width=300, height=200, datashade=True)
```

Error log:

```
KeyError Traceback (most recent call last)
File /opt/conda/envs/rhodes/lib/python3.10/site-packages/pandas/core/indexes/base.py:3802, in Index.get_loc(self, key, method, tolerance)
3801 try:
-> 3802 return self._engine.get_loc(casted_key)
3803 except KeyError as err:

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/pandas/_libs/index.pyx:138, in pandas._libs.index.IndexEngine.get_loc()

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/pandas/_libs/index.pyx:165, in pandas._libs.index.IndexEngine.get_loc()

File pandas/_libs/hashtable_class_helper.pxi:5745, in pandas._libs.hashtable.PyObjectHashTable.get_item()

File pandas/_libs/hashtable_class_helper.pxi:5753, in pandas._libs.hashtable.PyObjectHashTable.get_item()

KeyError: 'Variable'

The above exception was the direct cause of the following exception:

KeyError Traceback (most recent call last)
File /opt/conda/envs/rhodes/lib/python3.10/site-packages/IPython/core/formatters.py:972, in MimeBundleFormatter.__call__(self, obj, include, exclude)
969 method = get_real_method(obj, self.print_method)
971 if method is not None:
--> 972 return method(include=include, exclude=exclude)
973 return None
974 else:

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/dimension.py:1290, in Dimensioned._repr_mimebundle_(self, include, exclude)
1283 def _repr_mimebundle_(self, include=None, exclude=None):
1284 """
1285 Resolves the class hierarchy for the class rendering the
1286 object using any display hooks registered on Store.display
1287 hooks. The output of all registered display_hooks is then
1288 combined and returned.
1289 """
-> 1290 return Store.render(self)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/options.py:1425, in Store.render(cls, obj)
1423 data, metadata = {}, {}
1424 for hook in hooks:
-> 1425 ret = hook(obj)
1426 if ret is None:
1427 continue

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/ipython/display_hooks.py:279, in pprint_display(obj)
277 if not ip.display_formatter.formatters['text/plain'].pprint:
278 return None
--> 279 return display(obj, raw_output=True)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/ipython/display_hooks.py:250, in display(obj, raw_output, **kwargs)
248 elif isinstance(obj, (Layout, NdLayout, AdjointLayout)):
249 with option_state(obj):
--> 250 output = layout_display(obj)
251 elif isinstance(obj, (HoloMap, DynamicMap)):
252 with option_state(obj):

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/ipython/display_hooks.py:141, in display_hook..wrapped(element)
139 try:
140 max_frames = OutputSettings.options['max_frames']
--> 141 mimebundle = fn(element, max_frames=max_frames)
142 if mimebundle is None:
143 return {}, {}

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/ipython/display_hooks.py:215, in layout_display(layout, max_frames)
212 max_frame_warning(max_frames)
213 return None
--> 215 return render(layout)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/ipython/display_hooks.py:68, in render(obj, **kwargs)
65 if renderer.fig == 'pdf':
66 renderer = renderer.instance(fig='png')
---> 68 return renderer.components(obj, **kwargs)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/plotting/renderer.py:394, in Renderer.components(self, obj, fmt, comm, **kwargs)
391 embed = (not (dynamic or streams or self.widget_mode == 'live') or config.embed)
393 if embed or config.comms == 'default':
--> 394 return self._render_panel(plot, embed, comm)
395 return self._render_ipywidget(plot)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/plotting/renderer.py:401, in Renderer._render_panel(self, plot, embed, comm)
399 doc = Document()
400 with config.set(embed=embed):
--> 401 model = plot.layout._render_model(doc, comm)
402 if embed:
403 return render_model(model, comm)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/panel/viewable.py:509, in Renderable._render_model(self, doc, comm)
507 if comm is None:
508 comm = state._comm_manager.get_server_comm()
--> 509 model = self.get_root(doc, comm)
511 if config.embed:
512 embed_state(self, model, doc,
513 json=config.embed_json,
514 json_prefix=config.embed_json_prefix,
515 save_path=config.embed_save_path,
516 load_path=config.embed_load_path,
517 progress=False)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/panel/viewable.py:560, in Renderable.get_root(self, doc, comm, preprocess)
543 """
544 Returns the root model and applies pre-processing hooks
545
(...)
557 Returns the bokeh model corresponding to this panel object
558 """
559 doc = init_doc(doc)
--> 560 root = self._get_model(doc, comm=comm)
561 if preprocess:
562 self._preprocess(root)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/panel/layout/base.py:146, in Panel._get_model(self, doc, root, parent, comm)
144 if root is None:
145 root = model
--> 146 objects = self._get_objects(model, [], doc, root, comm)
147 props = dict(self._init_params(), objects=objects)
148 model.update(**self._process_param_change(props))

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/panel/layout/base.py:131, in Panel._get_objects(self, model, old_objects, doc, root, comm)
129 else:
130 try:
--> 131 child = pane._get_model(doc, root, model, comm)
132 except RerenderError:
133 return self._get_objects(model, current_objects[:i], doc, root, comm)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/panel/pane/holoviews.py:367, in HoloViews._get_model(self, doc, root, parent, comm)
365 plot = self.object
366 else:
--> 367 plot = self._render(doc, comm, root)
369 plot.pane = self
370 backend = plot.renderer.backend

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/panel/pane/holoviews.py:442, in HoloViews._render(self, doc, comm, root)
439 if comm:
440 kwargs['comm'] = comm
--> 442 return renderer.get_plot(self.object, **kwargs)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/plotting/bokeh/renderer.py:70, in BokehRenderer.get_plot(self_or_cls, obj, doc, renderer, **kwargs)
63 @bothmethod
64 def get_plot(self_or_cls, obj, doc=None, renderer=None, **kwargs):
65 """
66 Given a HoloViews Viewable return a corresponding plot instance.
67 Allows supplying a document attach the plot to, useful when
68 combining the bokeh model with another plot.
69 """
---> 70 plot = super().get_plot(obj, doc, renderer, **kwargs)
71 if plot.document is None:
72 plot.document = Document() if self_or_cls.notebook_context else curdoc()

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/plotting/renderer.py:213, in Renderer.get_plot(self_or_cls, obj, doc, renderer, comm, **kwargs)
210 raise SkipRendering(msg.format(dims=dims))
212 # Initialize DynamicMaps with first data item
--> 213 initialize_dynamic(obj)
215 if not renderer:
216 renderer = self_or_cls

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/plotting/util.py:257, in initialize_dynamic(obj)
255 continue
256 if not len(dmap):
--> 257 dmap[dmap._initial_key()]

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/spaces.py:1213, in DynamicMap.__getitem__(self, key)
1211 # Not a cross product and nothing cached so compute element.
1212 if cache is not None: return cache
-> 1213 val = self._execute_callback(*tuple_key)
1214 if data_slice:
1215 val = self._dataslice(val, data_slice)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/spaces.py:980, in DynamicMap._execute_callback(self, *args)
977 kwargs['_memoization_hash_'] = hash_items
979 with dynamicmap_memoization(self.callback, self.streams):
--> 980 retval = self.callback(*args, **kwargs)
981 return self._style(retval)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/spaces.py:580, in Callable.__call__(self, *args, **kwargs)
577 args, kwargs = (), dict(pos_kwargs, **kwargs)
579 try:
--> 580 ret = self.callable(*args, **kwargs)
581 except KeyError:
582 # KeyError is caught separately because it is used to signal
583 # invalid keys on DynamicMap and should not warn
584 raise

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/util/__init__.py:1011, in Dynamic._dynamic_operation..dynamic_operation(*key, **kwargs)
1009 def dynamic_operation(*key, **kwargs):
1010 key, obj = resolve(key, kwargs)
-> 1011 return apply(obj, *key, **kwargs)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/util/__init__.py:1003, in Dynamic._dynamic_operation..apply(element, *key, **kwargs)
1001 def apply(element, *key, **kwargs):
1002 kwargs = dict(util.resolve_dependent_kwargs(self.p.kwargs), **kwargs)
-> 1003 processed = self._process(element, key, kwargs)
1004 if (self.p.link_dataset and isinstance(element, Dataset) and
1005 isinstance(processed, Dataset) and processed._dataset is None):
1006 processed._dataset = element.dataset

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/util/__init__.py:985, in Dynamic._process(self, element, key, kwargs)
983 elif isinstance(self.p.operation, Operation):
984 kwargs = {k: v for k, v in kwargs.items() if k in self.p.operation.param}
--> 985 return self.p.operation.process_element(element, key, **kwargs)
986 else:
987 return self.p.operation(element, **kwargs)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/operation.py:194, in Operation.process_element(self, element, key, **params)
191 else:
192 self.p = param.ParamOverrides(self, params,
193 allow_extra_keywords=self._allow_extra_keywords)
--> 194 return self._apply(element, key)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/operation.py:141, in Operation._apply(self, element, key)
139 if not in_method:
140 element._in_method = True
--> 141 ret = self._process(element, key)
142 if hasattr(element, '_in_method') and not in_method:
143 element._in_method = in_method

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/operation/datashader.py:1594, in datashade._process(self, element, key)
1593 def _process(self, element, key=None):
-> 1594 agg = rasterize._process(self, element, key)
1595 shaded = shade._process(self, agg, key)
1596 return shaded

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/operation/datashader.py:1573, in rasterize._process(self, element, key)
1570 op = transform.instance(**{k:v for k,v in extended_kws.items()
1571 if k in transform.param})
1572 op._precomputed = self._precomputed
-> 1573 element = element.map(op, predicate)
1574 self._precomputed = op._precomputed
1576 unused_params = list(all_supplied_kws - all_allowed_kws)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/data/__init__.py:199, in PipelineMeta.pipelined..pipelined_fn(*args, **kwargs)
196 inst._in_method = True
198 try:
--> 199 result = method_fn(*args, **kwargs)
200 if PipelineMeta.disable:
201 return result

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/data/__init__.py:1207, in Dataset.map(self, *args, **kwargs)
1205 @wraps(LabelledData.map)
1206 def map(self, *args, **kwargs):
-> 1207 return super().map(*args, **kwargs)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/dimension.py:700, in LabelledData.map(self, map_fn, specs, clone)
698 return deep_mapped
699 else:
--> 700 return map_fn(self) if applies else self

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/operation.py:214, in Operation.__call__(self, element, **kwargs)
210 return element.clone([(k, self._apply(el, key=k))
211 for k, el in element.items()])
212 elif ((self._per_element and isinstance(element, Element)) or
213 (not self._per_element and isinstance(element, ViewableElement))):
--> 214 return self._apply(element)
215 elif 'streams' not in kwargs:
216 kwargs['streams'] = self.p.streams

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/core/operation.py:141, in Operation._apply(self, element, key)
139 if not in_method:
140 element._in_method = True
--> 141 ret = self._process(element, key)
142 if hasattr(element, '_in_method') and not in_method:
143 element._in_method = in_method

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/operation/datashader.py:491, in aggregate._process(self, element, key)
489 x, y, data, glyph = self._precomputed[element._plot_id]
490 else:
--> 491 x, y, data, glyph = self.get_agg_data(element, category)
493 if self.p.precompute:
494 self._precomputed[element._plot_id] = x, y, data, glyph

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/holoviews/operation/datashader.py:450, in aggregate.get_agg_data(cls, obj, category)
448 else:
449 df = paths[0] if paths else pd.DataFrame([], columns=[x.name, y.name])
--> 450 if category and df[category].dtype.name != 'category':
451 df[category] = df[category].astype('category')
453 is_custom = isinstance(df, dd.DataFrame) or cuDFInterface.applies(df)

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/pandas/core/frame.py:3807, in DataFrame.__getitem__(self, key)
3805 if self.columns.nlevels > 1:
3806 return self._getitem_multilevel(key)
-> 3807 indexer = self.columns.get_loc(key)
3808 if is_integer(indexer):
3809 indexer = [indexer]

File /opt/conda/envs/rhodes/lib/python3.10/site-packages/pandas/core/indexes/base.py:3804, in Index.get_loc(self, key, method, tolerance)
3802 return self._engine.get_loc(casted_key)
3803 except KeyError as err:
-> 3804 raise KeyError(key) from err
3805 except TypeError:
3806 # If we have a listlike key, _check_indexing_error will raise
3807 # InvalidIndexError. Otherwise we fall through and re-raise
3808 # the TypeError.
3809 self._check_indexing_error(key)

KeyError: 'Variable'
```

_Originally posted by @p4perf4ce in https://github.com/holoviz/hvplot/issues/486#issuecomment-1484087848_