Manually add Errorbar to Legend (ErrorbarContainer)

Question

I would like to add a manual legend to a matplotlib plot of this nature:

While I managed to add the horizontal bars to my legend, I am stuck with the creation of the horizontal error bars.

I based my approach on this StackOverflow answer.

import matplotlib.pyplot as plt

from matplotlib.patches import Patch
from matplotlib.container import ErrorbarContainer
from matplotlib.lines import Line2D
from matplotlib.collections import LineCollection

fig, ax = plt.subplots(
    figsize=(2, 2),
)

line = plt.Line2D(
    xdata = [],
    ydata = [],
    color = 'black',
    linestyle='none'
)
barline = LineCollection(
    segments = [
        [(0, 0), (0, 1)],
    ]
)
err = ErrorbarContainer(
    lines = (
        line,
        [line],
        [barline],
    ),
    has_xerr = True,
    has_yerr = False,
)

legend_elements = [
    Patch(facecolor='red', edgecolor='black', label='Average Warming'),
    Patch(facecolor='blue', edgecolor='black', label='Average Cooling'),
    err,
]

# Displaying the custom legend
fig.legend(
    handles=legend_elements,
    loc=10
)

Error:

---> 26 err = ErrorbarContainer(
     27     lines = (
     28         line,
     29         [line],
     30         [barline],
     31     ),
     32     has_xerr = True,
     33     has_yerr = False,
     34 )
     36 legend_elements = [
     37     Patch(facecolor='red', edgecolor='black', label='Average Warming'),
     38     Patch(facecolor='blue', edgecolor='black', label='Average Cooling'),
     39     err,
     40 ]
     42 # Displaying the custom legend

File /opt/homebrew/Caskroom/miniconda/base/envs/plotting/lib/python3.11/site-packages/matplotlib/container.py:18, in Container.__new__(cls, *args, **kwargs)
     17 def __new__(cls, *args, **kwargs):
---> 18     return tuple.__new__(cls, args[0])

IndexError: tuple index out of range

Should it not be enough to specify one line in the barlinecols parameter, since I am only interested in the horizontal (=x) error? After all, according to the documentation:

barlinecols : list of LineCollection with the horizontal and vertical error ranges.