BUG: xaxis formatter receives index instead of values

[gonzalobg]

Pandas version checks

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

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

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd
import matplotlib.pyplot as plt
from matplotlib.ticker import FuncFormatter

df = pd.DataFrame({'xs': [13.2, 42.2, 55.2], 'ys': [5.1, 7.1, 8.1]})

def formatter(value, pos):
    print(f'{value},{pos}')
    return value

ax = df.plot(kind='bar', x="xs", y="ys")
#ax.yaxis.set_major_formatter(FuncFormatter(formatter)) # This works, formatter gets yaxis value
ax.xaxis.set_major_formatter(FuncFormatter(formatter)) # This fails, formatter gets index as value

Issue Description

When using ax.yaxis.set_major_formatter with a FuncFormatter, the function taking a value and pos correctly receive the y-axis values as value.

When using an ax.xaxis.set_major_formatter, instead of getting the x-axis values, the function gets the index as value.

Expected Behavior

I'd expect the same behavior for both axis: the values get passed to value.

This behavior is currently correct for the y-axis, but incorrect for the x-axis.

Installed Versions

INSTALLED VERSIONS

commit : 66e3805
python : 3.8.16.final.0
python-bits : 64
OS : Linux
OS-release : 5.10.133+
Version : #1 SMP Fri Aug 26 08:44:51 UTC 2022
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.3.5
numpy : 1.21.6
pytz : 2022.6
dateutil : 2.8.2
pip : 21.1.3
setuptools : 57.4.0
Cython : 0.29.32
pytest : 3.6.4
hypothesis : None
sphinx : 1.8.6
blosc : None
feather : 0.4.1
xlsxwriter : None
lxml.etree : 4.9.2
html5lib : 1.0.1
pymysql : None
psycopg2 : 2.9.5 (dt dec pq3 ext lo64)
jinja2 : 2.11.3
IPython : 7.9.0
pandas_datareader: 0.9.0
bs4 : 4.6.3
bottleneck : None
fsspec : 2022.11.0
fastparquet : None
gcsfs : None
matplotlib : 3.2.2
numexpr : 2.8.4
odfpy : None
openpyxl : 3.0.10
pandas_gbq : 0.17.9
pyarrow : 9.0.0
pyxlsb : None
s3fs : None
scipy : 1.7.3
sqlalchemy : 1.4.45
tables : 3.7.0
tabulate : 0.8.10
xarray : 2022.12.0
xlrd : 1.2.0
xlwt : 1.3.0
numba : 0.56.4

[dicristina]

Try the example with an index other than [0, 1, 2] and you will see that your function will still get the values 0, 1, 2 because it does not get the DataFrame index at all. I don't think that this is an issue with pandas.

[mfloryan]

I don't know enough of the internals here but it seems that the same example works correctly when this line:

ax = df.plot(kind='bar', x="xs", y="ys")

is replaced with a direct call to matplotlib.

fig, ax = plt.subplots()
ax.bar(df['xs'], df['ys'])