I’ve been trying to get a horizontal line to show a threshold. It never worked. It gave me a mean-spirited error message that I couldn’t understand. I spent the last few days trying. I got this one:

Traceback (most recent call last):
  File "C:/Users/sandy_000/PycharmProjects/coffee_boss/viz/viz.py", line 65, in <module>
    df.plot(y=['diff'], ax=ax3)
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\pandas\plotting\_core.py", line 794, in __call__
    return plot_backend.plot(data, kind=kind, **kwargs)
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\pandas\plotting\_matplotlib\__init__.py", line 62, in plot
    plot_obj.generate()
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\pandas\plotting\_matplotlib\core.py", line 284, in generate
    self._adorn_subplots()
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\pandas\plotting\_matplotlib\core.py", line 472, in _adorn_subplots
    sharey=self.sharey,
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\pandas\plotting\_matplotlib\tools.py", line 316, in _handle_shared_axes
    _remove_labels_from_axis(ax.xaxis)
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\pandas\plotting\_matplotlib\tools.py", line 281, in _remove_labels_from_axis
    for t in axis.get_majorticklabels():
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\matplotlib\axis.py", line 1252, in get_majorticklabels
    ticks = self.get_major_ticks()
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\matplotlib\axis.py", line 1407, in get_major_ticks
    numticks = len(self.get_majorticklocs())
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\matplotlib\axis.py", line 1324, in get_majorticklocs
    return self.major.locator()
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\matplotlib\dates.py", line 1431, in __call__
    self.refresh()
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\matplotlib\dates.py", line 1451, in refresh
    dmin, dmax = self.viewlim_to_dt()
  File "C:\Users\sandy_000\venv\coffee_boss\lib\site-packages\matplotlib\dates.py", line 1202, in viewlim_to_dt
    .format(vmin))
ValueError: view limit minimum 0.0 is less than 1 and is an invalid Matplotlib date value. This often happens if you pass a non-datetime value to an axis that has datetime units

So what I did was change

ax3.axhline(threshold, linewidth=1, color='r')

df.plot(y=[small_window['name'], 'thresholded'], secondary_y=['thresholded'], ax=ax1)
df.plot(y=[small_window['name']], ax=ax2)
df.plot(y=['diff'], ax=ax3)

To

df.plot(y=[small_window['name'], 'thresholded'], secondary_y=['thresholded'], ax=ax1)
df.plot(y=[small_window['name']], ax=ax2)
df.plot(y=['diff'], ax=ax3)

ax3.axhline(threshold, linewidth=1, color='r')

Yes. Same, but the hline happens after the plot. OK, I can make the intuitive leap for why this works and not be cross about it, but I wish I’d tried this a week ago.