REGR: Sorted pandas series is not plotted in the expected order in pandas 1.2.0

[GajjarMihir]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

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Code Sample

Note: The code was executed in Jupyter notebook.

import pandas as pd
print(pd.__version__)
d = {'col1': [4,2,3,1]}
df = pd.DataFrame(data=d)
df['col1'].sort_values().plot(kind='bar')

Problem description

In Pandas 1.2.0, the sorted values are not plotted in the correct order. Note that df['col1'].sort_values() does return sorted values.

Expected Output

In Pandas 1.1.5, this works as expected. This might be the case in older versions of pandas as well.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : b5958ee
python : 3.8.5.final.0
python-bits : 64
OS : Darwin
OS-release : 20.2.0
Version : Darwin Kernel Version 20.2.0: Wed Dec 2 20:39:59 PST 2020; root:xnu-7195.60.75~1/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_CA.UTF-8
LOCALE : en_CA.UTF-8

pandas : 1.1.5
numpy : 1.19.1
pytz : 2020.1
dateutil : 2.8.1
pip : 20.2.4
setuptools : 50.3.0.post20201006
Cython : 0.29.21
pytest : 6.1.1
hypothesis : 5.37.4
sphinx : 3.2.1
blosc : None
feather : None
xlsxwriter : 1.3.7
lxml.etree : 4.6.1
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.18.1
pandas_datareader: None
bs4 : 4.9.3
bottleneck : 1.3.2
fsspec : 0.8.3
fastparquet : None
gcsfs : None
matplotlib : 3.3.2
numexpr : 2.7.1
odfpy : None
openpyxl : 3.0.5
pandas_gbq : None
pyarrow : None
pytables : None
pyxlsb : None
s3fs : None
scipy : 1.5.0
sqlalchemy : 1.3.19
tables : 3.6.1
tabulate : None
xarray : None
xlrd : 1.2.0
xlwt : 1.3.0
numba : 0.51.2

[jreback]

pls edit the title to reflect the issue

[GajjarMihir]

pls edit the title to reflect the issue

Just fixed this. Thanks :)

[mzeitlin11]

Thanks for the report @GajjarMihir! This is a regression caused by #28733. Other regression from this was (#38736)

[simonjayhawkins]

cc @nrebena

[nonZero]

Another simple reproduction of this undocumented change in behavior:

import pandas as pd
pd.Series([10, 50, 20, 30], index=[1910, 1920, 1980, 1950]).plot.bar(title=pd.__version__);

[simonjayhawkins]

This is a regression caused by #28733.

#28733 has been reverted. needs tests to prevent regressions.

[avinashpancham]

take