DataFrame.plot.box ignores by argument

[ischurov]

Code Sample, a copy-pastable example if possible

df = pd.DataFrame({'a': ['one', 'one', 'two', 'one', 'two', 'two'], 
                   'b': [1, 2, -1, 2, -2, -3]})
df.boxplot(by='a')
# two boxplots, one for 'one' and the other for 'two'
# this is an expected behaviour

df.plot(kind='box', by='a')
# one boxplot, grouping ignored

df.plot.box(by='a')
# one boxplot, grouping ignored

Problem description

It seems that by argument is silently ignored and no grouping occurs. Probably, it is related to #9274.

Expected Output

I expect that three figures will be the same as the first one.

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.5.2.final.0 python-bits: 64 OS: Darwin OS-release: 16.3.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: en_US.UTF-8 LANG: en_US.UTF-8 LOCALE: en_US.UTF-8

pandas: 0.19.0+311.gb895968
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.1
numpy: 1.11.2
scipy: None
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: 1.5.1
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 1.5.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
pandas_datareader: None

[bkandel]

This is also related to #7998. I thought that the code for DataFrame.boxplot, DataFrame.plot(kind=box), and DataFrame.plot.box were the same, but apparently not. It looks like the by arg was never implemented for the plot(kind=box) method: https://github.com/pandas-dev/pandas/blob/master/pandas/tools/plotting.py#L2794 doesn't have any counterpart in https://github.com/pandas-dev/pandas/blob/master/pandas/tools/plotting.py#L2287.

[TomAugspurger]

@ischurov is this any different that #9274? It seems like the same issue to me.

[bkandel]

I don't think it's the same -- from the discussion there, it looks like #9274 is about documenting that by only works when kind='box'. But here the bug report is that by doesn't actually work when kind='box'. So this is a bug report, not a request for additional documentation.

[ischurov]

@TomAugspurger , I agree with @bkandel 's comment.

[blokeley]

The Stack Overflow question got no answers. It seems to be a bug.

[jorisvandenbossche]

I actually think this is the same issue. by is documented for several df.plot.<methods> like df.plot.box, df.plot.hist. But, this was never implemented and only the df.hist and df.boxplot methods actually support this (those methods have a complete separate implementation).

Ideally on the long term the implementation of df.hist and df.boxplot would get integrated into the df.plot. kinds, and then they could support by, but for now, let's fix the documentation.

[Jeitan]

@jorisvandenbossche I would LOVE to see by implemented in plot.hist and have made a comment along those lines at #28177 . Seeing as how that's what the documentation has been saying all along...