Maximum value not always included in pcolormesh output when levels are requested #10911
Description
What happened?
When plotting data with "pcolormesh" and requesting levels, if the calculated "vmax" if equal to the upper bound of the last bin, it is not included in the map, as if it was "over" or "missing".
What did you expect to happen?
I expected values == vmax to be painted with the color of the last bin, which is what contourf does.
Minimal Complete Verifiable Example
# /// script
# requires-python = ">=3.11"
# dependencies = [
# "xarray[complete]@git+https://github.com/pydata/xarray.git@main",
# ]
# ///
#
# This script automatically imports the development branch of xarray to check for issues.
# Please delete this header if you have _not_ tested this script with `uv run`!
import xarray as xr
xr.show_versions()
# your reproducer code ...
import xarray as xr
ds = xr.tutorial.open_dataset('air_temperature')
air = ds.air.isel(time=0).round(-1)
air.plot(levels=5)Steps to reproduce
The example above gives the image below:
You can see that with the heavy discretization I imposed, the values at the bottom are equal to the calculated "vmax" and are not shown.
MVCE confirmation
- Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
- Complete example — the example is self-contained, including all data and the text of any traceback.
- Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
- New issue — a search of GitHub Issues suggests this is not a duplicate.
- Recent environment — the issue occurs with the latest version of xarray and its dependencies.
Relevant log output
Anything else we need to know?
Matplotlib's from_levels_and_colors, it is said that :
The quantization levels used to construct the BoundaryNorm. Value v is quantized to level i if lev[i] <= v < lev[i+1].
So this explains why xarray's implementation gives no color. However, the result seems to be unexpected and incoherent with the output of contourf. I think it is logic to expect the maximum value to be included in the plot when simply passing levels as a scalar.
Environment
INSTALLED VERSIONS
commit: None
python: 3.13.3 | packaged by conda-forge | (main, Apr 14 2025, 20:44:03) [GCC 13.3.0]
python-bits: 64
OS: Linux
OS-release: 6.17.4-200.fc42.x86_64
machine: x86_64
processor:
byteorder: little
LC_ALL: None
LANG: fr_CA.UTF-8
LOCALE: ('fr_CA', 'UTF-8')
libhdf5: 1.14.6
libnetcdf: 4.9.2
xarray: 2025.10.2.dev18+ge49cfc4f2
pandas: 2.2.3
numpy: 2.2.6
scipy: 1.16.1
netCDF4: 1.7.2
pydap: None
h5netcdf: 1.6.1
h5py: 3.13.0
zarr: None
cftime: 1.6.4
nc_time_axis: 1.4.1
iris: None
bottleneck: 1.5.0
dask: 2025.4.1
distributed: 2025.4.1
matplotlib: 3.10.3
cartopy: None
seaborn: None
numbagg: 0.9.0
fsspec: 2025.3.2
cupy: None
pint: 0.25
sparse: None
flox: 0.10.5
numpy_groupies: 0.11.2
setuptools: 80.1.0
pip: 25.1.1
conda: None
pytest: 8.3.5
mypy: 1.15.0
IPython: 9.2.0
sphinx: 8.2.3