BUG: `SeriesGroupBy.plot` does not respect `cmap` parameter

[matteosantama]

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Reproducible Example

import pandas as pd

chunk = pd.DataFrame(index=[0, 1, 2])

df = pd.concat([chunk.assign(y=i, i=i) for i in range(3)])

df.groupby("i")["y"].plot(legend=True, cmap="tab20");

Issue Description

The above code snippet demonstrates that cmap="tab20" is not respected. The parameter is respected in DataFrame.plot, which can be checked by viewing

df.plot(cmap="tab20")

Expected Behavior

The cmap parameter should be correctly handled

Installed Versions

INSTALLED VERSIONS

commit : a671b5a
python : 3.11.2.final.0
python-bits : 64
OS : Darwin
OS-release : 23.1.0
Version : Darwin Kernel Version 23.1.0: Mon Oct 9 21:27:24 PDT 2023; root:xnu-10002.41.9~6/RELEASE_ARM64_T6000
machine : arm64
processor : arm
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.1.4
numpy : 1.26.2
pytz : 2023.3.post1
dateutil : 2.8.2
setuptools : 68.2.2
pip : 23.3.1
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.19.0
pandas_datareader : None
bs4 : 4.12.2
bottleneck : None
dataframe-api-compat: None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.8.2
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.11.4
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.4
qtpy : None
pyqt5 : None

[zhuoyahuang]

take

[zhuoyahuang]

Hi @matteosantama Can you please attach a screenshot if this is still an issue?
I couldn't reproduce the problem, I've tried a few different colormaps and the cmap feature looks good to me

[matteosantama]

You can see the problem in your screenshot. In your first plot, all three lines have the same color. They should have three different colors, corresponding to the three group-by keys.

For example, see what you get if you don't pass any colormap

df.groupby("i")["y"].plot(legend=True);

You should be able to control the colors used.

Just a heads up, though, it looks like the right parameter is colormap, not cmap: https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.SeriesGroupBy.plot.html#pandas.core.groupby.SeriesGroupBy.plot

The problem persists with the right parameter, though.