Count aggregation on object column failing on heatmap

ALL software version info

holoviews 1.15.2
hvplot 0.8.1
pandas 1.5.1
python 3.9.13
Macbook Pro M1 (macOS 12.6).

Description of expected behavior and the observed behavior

While I am using hvplot to plot, after updating to holoviews 1.15.2 one of my plots, a heat map plot with a count aggregation on a object column, failed to process and instead resulted in the error I have included below. This does not happen on 1.15.1.

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

I have created a minimal situation to reproduce the error I am seeing on my plot. Essentially I am generating a heatmap with a count aggregation on a string (object) column of a data frame. In the example below it will always come out to 4 for each cell.

If I change the count from the object column id to the int column value then the plot is created as expected.

import hvplot.pandas  # noqa
import pandas as pd
import random

def gen_df():
    df = pd.DataFrame({
        'month': dates.month,
        'hour': dates.hour,
        'day': dates.day,
        'value': [random.randint(0, 100) for i in range(len(dates))],
    })

    df['id'] = 'test'

    return df

dates = pd.date_range("2022-09-01", "2022-10-31", freq='h')

df = gen_df()

for i in range(3):
    df = pd.concat([df, gen_df()])

df.hvplot.heatmap(x='hour', y='day', C='id', groupby='month', reduce_function='count')

Stack traceback and/or browser JavaScript console output

Traceback (most recent call last):
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/IPython/core/formatters.py", line 222, in catch_format_error
    r = method(self, *args, **kwargs)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/IPython/core/formatters.py", line 973, in __call__
    return method(include=include, exclude=exclude)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/core/dimension.py", line 1293, in _repr_mimebundle_
    return Store.render(self)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/core/options.py", line 1426, in render
    ret = hook(obj)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/ipython/display_hooks.py", line 277, in pprint_display
    return display(obj, raw_output=True)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/ipython/display_hooks.py", line 253, in display
    output = map_display(obj)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/ipython/display_hooks.py", line 141, in wrapped
    mimebundle = fn(element, max_frames=max_frames)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/ipython/display_hooks.py", line 201, in map_display
    return render(vmap)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/ipython/display_hooks.py", line 68, in render
    return renderer.components(obj, **kwargs)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/plotting/renderer.py", line 398, in components
    return self._render_panel(plot, embed, comm)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/plotting/renderer.py", line 405, in _render_panel
    model = plot.layout._render_model(doc, comm)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/panel/viewable.py", line 508, in _render_model
    model = self.get_root(doc, comm)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/panel/viewable.py", line 559, in get_root
    root = self._get_model(doc, comm=comm)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/panel/layout/base.py", line 146, in _get_model
    objects = self._get_objects(model, [], doc, root, comm)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/panel/layout/base.py", line 131, in _get_objects
    child = pane._get_model(doc, root, model, comm)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/panel/pane/holoviews.py", line 265, in _get_model
    plot = self._render(doc, comm, root)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/panel/pane/holoviews.py", line 342, in _render
    return renderer.get_plot(self.object, **kwargs)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/plotting/bokeh/renderer.py", line 70, in get_plot
    plot = super().get_plot(obj, doc, renderer, **kwargs)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/plotting/renderer.py", line 240, in get_plot
    plot.update(init_key)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/plotting/plot.py", line 948, in update
    return self.initialize_plot()
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/plotting/bokeh/element.py", line 1380, in initialize_plot
    ranges = self.compute_ranges(self.hmap, key, ranges)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/plotting/plot.py", line 607, in compute_ranges
    self._compute_group_range(group, elements, ranges, framewise,
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/plotting/plot.py", line 717, in _compute_group_range
    data_range = el.range(el_dim, dimension_range=False)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/core/data/__init__.py", line 204, in pipelined_fn
    result = method_fn(*args, **kwargs)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/element/raster.py", line 853, in range
    self.gridded._binned = False
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/element/raster.py", line 819, in gridded
    self._gridded = categorical_aggregate2d(self)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/param/parameterized.py", line 3654, in __new__
    return inst.__call__(*args,**params)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/core/operation.py", line 220, in __call__
    return element.apply(self, **kwargs)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/core/accessors.py", line 45, in pipelined_call
    result = __call__(*args, **kwargs)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/core/accessors.py", line 202, in __call__
    new_obj = apply_function(self._obj, **inner_kwargs)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/core/operation.py", line 214, in __call__
    return self._apply(element)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/core/operation.py", line 141, in _apply
    ret = self._process(element, key)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/holoviews/element/util.py", line 229, in _process
    raise ValueError("Must have at two dimensions to aggregate over"
ValueError: Must have at two dimensions to aggregate overand one value dimension to aggregate on.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/IPython/core/interactiveshell.py", line 1997, in showtraceback
    stb = self.InteractiveTB.structured_traceback(
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/IPython/core/ultratb.py", line 1112, in structured_traceback
    return FormattedTB.structured_traceback(
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/IPython/core/ultratb.py", line 1006, in structured_traceback
    return VerboseTB.structured_traceback(
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/IPython/core/ultratb.py", line 859, in structured_traceback
    formatted_exception = self.format_exception_as_a_whole(etype, evalue, etb, number_of_lines_of_context,
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/IPython/core/ultratb.py", line 812, in format_exception_as_a_whole
    frames.append(self.format_record(r))
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/IPython/core/ultratb.py", line 730, in format_record
    result += ''.join(_format_traceback_lines(frame_info.lines, Colors, self.has_colors, lvals))
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/stack_data/utils.py", line 145, in cached_property_wrapper
    value = obj.__dict__[self.func.__name__] = self.func(obj)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/stack_data/core.py", line 734, in lines
    pieces = self.included_pieces
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/stack_data/utils.py", line 145, in cached_property_wrapper
    value = obj.__dict__[self.func.__name__] = self.func(obj)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/stack_data/core.py", line 677, in included_pieces
    scope_pieces = self.scope_pieces
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/stack_data/utils.py", line 145, in cached_property_wrapper
    value = obj.__dict__[self.func.__name__] = self.func(obj)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/stack_data/core.py", line 617, in scope_pieces
    for piece in self.source.pieces
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/stack_data/utils.py", line 145, in cached_property_wrapper
    value = obj.__dict__[self.func.__name__] = self.func(obj)
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/stack_data/core.py", line 106, in pieces
    return list(self._clean_pieces())
  File "/Users/adamstewart/miniforge3/envs/light-command-dash/lib/python3.9/site-packages/stack_data/core.py", line 130, in _clean_pieces
    raise AssertionError("Pieces mismatches: %s" % mismatches)
AssertionError: Pieces mismatches: [{680, 681}, {696, 695}, {708, 709}, {714, 715}]

Screenshots or screencasts of the bug in action

Expected output:

holoviz / holoviews