Curve steps interpolation on string-valued/categorical kdims is not possible

[remisphere]

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Software version info

• OS: Ubuntu 20.04.4 LTS x86_64
  • environment: miniconda3-4.7.12
    • Python: 3.9.7 - holoviews: 1.14.8 - bokeh: 2.4.1 - pandas: 1.4.2

Description of expected behavior and the observed behavior

While plotting a curve with a string or categorical independent variable is possible, as shown in the user guide, the render fails when using the 'steps' interpolation modes. The error seems to be the same for string and categorical types, and the error is either a TypeError when using 'steps-mid' interpolation mode or a ValueError when using 'steps-pre' or 'steps-post' modes.

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

"""With string kdim."""

import pandas as pd
import holoviews as hv


hv.extension('bokeh')

df = pd.DataFrame.from_dict({'int': [1, 2], 'string': ['a', 'b']})
curve = hv.Curve(df, 'string', 'int')
curve.opts(interpolation='steps-mid')

hv.render(curve)

"""With categorical kdim. Behaves the same as string kdims."""

import pandas as pd
import holoviews as hv


hv.extension('bokeh')

df = pd.DataFrame.from_dict({'int': [1, 2], 'string': ['a', 'b']})
df['string'] = df['string'].astype('category')
curve = hv.Curve(df, 'string', 'int')
curve.opts(interpolation='steps-mid')

hv.render(curve)

"""With 'steps-pre' instead of 'steps-mid', gives a different error."""

import pandas as pd
import holoviews as hv


hv.extension('bokeh')

df = pd.DataFrame.from_dict({'int': [1, 2], 'string': ['a', 'b']})
curve = hv.Curve(df, 'string', 'int')
curve.opts(interpolation='steps-pre')

hv.render(curve)

Stack tracebacks

'steps-mid' interpolation

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/util/__init__.py", line 857, in render
    plot = renderer_obj.get_plot(obj)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/bokeh/renderer.py", line 73, in get_plot
    plot = super(BokehRenderer, self_or_cls).get_plot(obj, doc, renderer, **kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/renderer.py", line 243, in get_plot
    plot.update(init_key)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/plot.py", line 991, in update
    return self.initialize_plot()
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/bokeh/element.py", line 1418, in initialize_plot
    self._init_glyphs(plot, element, ranges, source)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/bokeh/element.py", line 1362, in _init_glyphs
    data, mapping, style = self.get_data(element, ranges, style)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/bokeh/chart.py", line 363, in get_data
    element = interpolate_curve(element, interpolation=self.interpolation)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/param/parameterized.py", line 3652, in __new__
    return inst.__call__(*args,**params)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/operation.py", line 220, in __call__
    return element.apply(self, **kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/accessors.py", line 47, in pipelined_call
    result = __call__(*args, **kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/accessors.py", line 204, in __call__
    new_obj = apply_function(self._obj, **inner_kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/operation.py", line 214, in __call__
    return self._apply(element)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/operation.py", line 141, in _apply
    ret = self._process(element, key)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/operation/element.py", line 988, in _process
    return element.map(self._process_layer, Element)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/data/__init__.py", line 207, in pipelined_fn
    result = method_fn(*args, **kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/data/__init__.py", line 1223, in map
    return super(Dataset, self).map(*args, **kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/dimension.py", line 710, in map
    return map_fn(self) if applies else self
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/operation/element.py", line 982, in _process_layer
    xs, dvals = INTERPOLATE_FUNCS[self.p.interpolation](x, dvals)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/operation/element.py", line 943, in pts_to_midstep
    steps[1:-1:2] = steps[2::2] = x[:-1] + (x[1:] - x[:-1])/2
TypeError: unsupported operand type(s) for -: 'str' and 'str'

'steps-pre' interpolation (same error as 'step-post')

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/util/__init__.py", line 857, in render
    plot = renderer_obj.get_plot(obj)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/bokeh/renderer.py", line 73, in get_plot
    plot = super(BokehRenderer, self_or_cls).get_plot(obj, doc, renderer, **kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/renderer.py", line 243, in get_plot
    plot.update(init_key)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/plot.py", line 991, in update
    return self.initialize_plot()
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/bokeh/element.py", line 1418, in initialize_plot
    self._init_glyphs(plot, element, ranges, source)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/bokeh/element.py", line 1362, in _init_glyphs
    data, mapping, style = self.get_data(element, ranges, style)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/plotting/bokeh/chart.py", line 363, in get_data
    element = interpolate_curve(element, interpolation=self.interpolation)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/param/parameterized.py", line 3652, in __new__
    return inst.__call__(*args,**params)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/operation.py", line 220, in __call__
    return element.apply(self, **kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/accessors.py", line 47, in pipelined_call
    result = __call__(*args, **kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/accessors.py", line 204, in __call__
    new_obj = apply_function(self._obj, **inner_kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/operation.py", line 214, in __call__
    return self._apply(element)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/operation.py", line 141, in _apply
    ret = self._process(element, key)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/operation/element.py", line 988, in _process
    return element.map(self._process_layer, Element)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/data/__init__.py", line 207, in pipelined_fn
    result = method_fn(*args, **kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/data/__init__.py", line 1223, in map
    return super(Dataset, self).map(*args, **kwargs)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/core/dimension.py", line 710, in map
    return map_fn(self) if applies else self
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/operation/element.py", line 982, in _process_layer
    xs, dvals = INTERPOLATE_FUNCS[self.p.interpolation](x, dvals)
  File "/home/REDACTED/.pyenv/versions/miniconda3-4.7.12/envs/REDACTED/lib/python3.9/site-packages/holoviews/operation/element.py", line 927, in pts_to_prestep
    steps[0::2] = x
ValueError: could not convert string to float: 'a'