Overlay of HLines with categorical axis doesn't work.

[BMM3]

ALL software version info

holoviews: 1.18.3
pandas: 2.2.1
bokeh: 3.4.0

Description of expected behavior and the observed behavior

When creating an overlay of a HLines graph with a categorical axes (e.g. boxwhisker), I get an error.
Creating an overlay with a continuous axis (e.g. Scatter) does work normally.
Creating an overly of HLine with a categorical axis also works normally.

Complete, minimal, self-contained example code that reproduces the issue

import pandas as pd
import holoviews as hv
df = pd.DataFrame({'x':  [1,1,1,1,2,2,2,2], 
                   'y': [2,3,4,5,4,5,3,7],
                   })

# HLines gives an issue
p1 = hv.BoxWhisker(df, 'x', 'y')
p1_ref = hv.HLines([5])
p1_total = p1*p1_ref

# HLine does work
p2 = hv.BoxWhisker(df, 'x', 'y')
p2_ref = hv.HLine(5)
p2_total = p2*p2_ref

Stack traceback and/or browser JavaScript console output

---

ValueError Traceback (most recent call last)
File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\IPython\core\formatters.py:977, in MimeBundleFormatter.call(self, obj, include, exclude)
974 method = get_real_method(obj, self.print_method)
976 if method is not None:
--> 977 return method(include=include, exclude=exclude)
978 return None
979 else:

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\core\dimension.py:1286, in Dimensioned.repr_mimebundle(self, include, exclude)
1279 def repr_mimebundle(self, include=None, exclude=None):
1280 """
1281 Resolves the class hierarchy for the class rendering the
1282 object using any display hooks registered on Store.display
1283 hooks. The output of all registered display_hooks is then
1284 combined and returned.
1285 """
-> 1286 return Store.render(self)

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\core\options.py:1428, in Store.render(cls, obj)
1426 data, metadata = {}, {}
1427 for hook in hooks:
-> 1428 ret = hook(obj)
1429 if ret is None:
1430 continue

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\ipython\display_hooks.py:287, in pprint_display(obj)
285 if not ip.display_formatter.formatters['text/plain'].pprint:
286 return None
--> 287 return display(obj, raw_output=True)

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\ipython\display_hooks.py:255, in display(obj, raw_output, **kwargs)
253 elif isinstance(obj, (CompositeOverlay, ViewableElement)):
254 with option_state(obj):
--> 255 output = element_display(obj)
256 elif isinstance(obj, (Layout, NdLayout, AdjointLayout)):
257 with option_state(obj):

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\ipython\display_hooks.py:149, in display_hook..wrapped(element)
147 try:
148 max_frames = OutputSettings.options['max_frames']
--> 149 mimebundle = fn(element, max_frames=max_frames)
150 if mimebundle is None:
151 return {}, {}

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\ipython\display_hooks.py:195, in element_display(element, max_frames)
192 if type(element) not in Store.registry[backend]:
193 return None
--> 195 return render(element)

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\ipython\display_hooks.py:76, in render(obj, **kwargs)
73 if renderer.fig == 'pdf':
74 renderer = renderer.instance(fig='png')
---> 76 return renderer.components(obj, **kwargs)

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\plotting\renderer.py:396, in Renderer.components(self, obj, fmt, comm, **kwargs)
393 embed = (not (dynamic or streams or self.widget_mode == 'live') or config.embed)
395 if embed or config.comms == 'default':
--> 396 return self._render_panel(plot, embed, comm)
397 return self._render_ipywidget(plot)

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\plotting\renderer.py:403, in Renderer._render_panel(self, plot, embed, comm)
401 doc = Document()
402 with config.set(embed=embed):
--> 403 model = plot.layout._render_model(doc, comm)
404 if embed:
405 return render_model(model, comm)

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\panel\viewable.py:736, in Viewable._render_model(self, doc, comm)
734 if comm is None:
735 comm = state._comm_manager.get_server_comm()
--> 736 model = self.get_root(doc, comm)
738 if self._design and self._design.theme.bokeh_theme:
739 doc.theme = self._design.theme.bokeh_theme

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\panel\layout\base.py:320, in Panel.get_root(self, doc, comm, preprocess)
316 def get_root(
317 self, doc: Optional[Document] = None, comm: Optional[Comm] = None,
318 preprocess: bool = True
319 ) -> Model:
--> 320 root = super().get_root(doc, comm, preprocess)
321 # ALERT: Find a better way to handle this
322 if hasattr(root, 'styles') and 'overflow-x' in root.styles:

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\panel\viewable.py:667, in Renderable.get_root(self, doc, comm, preprocess)
665 wrapper = self._design._wrapper(self)
666 if wrapper is self:
--> 667 root = self._get_model(doc, comm=comm)
668 if preprocess:
669 self._preprocess(root)

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\panel\layout\base.py:186, in Panel._get_model(self, doc, root, parent, comm)
184 root = root or model
185 self._models[root.ref['id']] = (model, parent)
--> 186 objects, _ = self._get_objects(model, [], doc, root, comm)
187 props = self._get_properties(doc)
188 props[self._property_mapping['objects']] = objects

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\panel\layout\base.py:168, in Panel._get_objects(self, model, old_objects, doc, root, comm)
166 else:
167 try:
--> 168 child = pane._get_model(doc, root, model, comm)
169 except RerenderError as e:
170 if e.layout is not None and e.layout is not self:

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\panel\pane\holoviews.py:429, in HoloViews._get_model(self, doc, root, parent, comm)
427 plot = self.object
428 else:
--> 429 plot = self._render(doc, comm, root)
431 plot.pane = self
432 backend = plot.renderer.backend

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\panel\pane\holoviews.py:525, in HoloViews._render(self, doc, comm, root)
522 if comm:
523 kwargs['comm'] = comm
--> 525 return renderer.get_plot(self.object, **kwargs)

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\plotting\bokeh\renderer.py:68, in BokehRenderer.get_plot(self_or_cls, obj, doc, renderer, **kwargs)
61 @bothmethod
62 def get_plot(self_or_cls, obj, doc=None, renderer=None, **kwargs):
63 """
64 Given a HoloViews Viewable return a corresponding plot instance.
65 Allows supplying a document attach the plot to, useful when
66 combining the bokeh model with another plot.
67 """
---> 68 plot = super().get_plot(obj, doc, renderer, **kwargs)
69 if plot.document is None:
70 plot.document = Document() if self_or_cls.notebook_context else curdoc()

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\plotting\renderer.py:240, in Renderer.get_plot(self_or_cls, obj, doc, renderer, comm, **kwargs)
237 defaults = [kd.default for kd in plot.dimensions]
238 init_key = tuple(v if d is None else d for v, d in
239 zip(plot.keys[0], defaults))
--> 240 plot.update(init_key)
241 else:
242 plot = obj

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\plotting\plot.py:955, in DimensionedPlot.update(self, key)
953 def update(self, key):
954 if len(self) == 1 and key in (0, self.keys[0]) and not self.drawn:
--> 955 return self.initialize_plot()
956 item = self.getitem(key)
957 self.traverse(lambda x: setattr(x, '_updated', True))

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\plotting\bokeh\element.py:2892, in OverlayPlot.initialize_plot(self, ranges, plot, plots)
2890 if plot and not self.overlaid:
2891 self._update_plot(key, plot, element)
-> 2892 self._update_ranges(element, ranges)
2894 panels = []
2895 for key, subplot in self.subplots.items():

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\plotting\bokeh\element.py:1048, in ElementPlot._update_ranges(self, element, ranges)
1045 y_range = self.handles['y_range']
1046 plot = self.handles['plot']
-> 1048 self._update_main_ranges(element, x_range, y_range, ranges)
1050 if self._subcoord_overlaid:
1051 return

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\plotting\bokeh\element.py:1091, in ElementPlot._update_main_ranges(self, element, x_range, y_range, ranges)
1089 xfactors, yfactors = None, None
1090 if any(isinstance(ax_range, FactorRange) for ax_range in [x_range, y_range]):
-> 1091 xfactors, yfactors = self._get_factors(element, ranges)
1092 framewise = self.framewise
1093 streaming = (self.streaming and any(stream._triggering and stream.following
1094 for stream in self.streaming))

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\plotting\bokeh\element.py:2845, in OverlayPlot._get_factors(self, overlay, ranges)
2843 if el is not None:
2844 elranges = util.match_spec(el, ranges)
-> 2845 xfs, yfs = sp._get_factors(el, elranges)
2846 if len(xfs):
2847 xfactors.append(xfs)

File c:\Users\bmm\Documents\projects\git\mlxscripts.venv\Lib\site-packages\holoviews\plotting\bokeh\element.py:1461, in ElementPlot._get_factors(self, element, ranges)
1457 def _get_factors(self, element, ranges):
1458 """
1459 Get factors for categorical axes.
1460 """
-> 1461 xdim, ydim = element.dimensions()[:2]
1462 xvals = self._get_dimension_factors(element, ranges, xdim)
1463 yvals = self._get_dimension_factors(element, ranges, ydim)

ValueError: not enough values to unpack (expected 2, got 1)