Bug summary

Fonts can'be read on IBM USS (unix) so no labeling possible a raw graph can be created and saved to file.

Python: 3.12.13 (heads/pyz_dev-3.12:fe57f277d86, May 24 2026, 08:17:10) [Clang 14.0.0 (build 588d20f)]
Matplotlib: 3.8.4
Matplotlib module: /usr/local/pip/matplotlib/init.py
FT2Font module: /usr/local/pip/matplotlib/ft2font.cpython-312.so
FreeType: 2.6.1
Matplotlib data: /usr/local/pip/matplotlib/mpl-data

font content look ok:
chtag -p "/usr/local/pip/matplotlib/mpl-data/fonts/ttf/DejaVuSans.ttf"
file "/usr/local/pip/matplotlib/mpl-data/fonts/ttf/DejaVuSans.ttf"
od -An -tx1 -N16 "/usr/local/pip/matplotlib/mpl-data/fonts/ttf/DejaVuSans.ttf"

• untagged T=off /usr/local/pip/matplotlib/mpl-data/fonts/ttf/DejaVuSans.ttf
  file: magic file "/etc/magic": EDC5129I No such file or directory.
  00 01 00 00 00 14 01 00 00 04 00 40 46 46 54 4D

S@ZOS31 /u/s/tlh_analyse_try > ls -l "/usr/local/pip/matplotlib/mpl-data/fonts/ttf/DejaVuSans.ttf"
-rwxr-xr-x 1 ROOT SUDOGRP 756072 Feb 9 2026 /usr/local/pip/matplotlib/mpl-data/fonts/ttf/DejaVuSans.ttf

Code for reproduction

python3 - <<'PY'
import glob
from matplotlib.ft2font import FT2Font

font_dir = "/usr/local/pip/matplotlib/mpl-data/fonts/ttf"
for path in glob.glob(font_dir + "/*.ttf"):
    try:
        FT2Font(path)
        print("OK:   ", path)
    except Exception as exc:
        print("FAIL: ", path, exc)
PY


-------------------- smoke test for a plot
import os

import matplotlib

matplotlib.use("Agg")
import matplotlib.pyplot as plt
from matplotlib import font_manager


output_path = os.path.join(os.path.dirname(os.path.abspath(__file__)), "matplotlib_uss_test.png")
font_path = font_manager.findfont("DejaVu Sans")

print(f"Matplotlib backend: {matplotlib.get_backend()}")
print(f"Selected font: {font_path}")
print(f"Font exists: {os.path.isfile(font_path)}")

plt.figure(figsize=(6, 4))
plt.plot([0, 1, 2, 3], [0, 1, 4, 9], marker="o")
plt.title("Matplotlib USS smoke test")
plt.xlabel("X")
plt.ylabel("Y")
plt.grid(True)
plt.savefig(output_path, dpi=100)
plt.close()

print(f"Plot saved: {output_path}")
print(f"File size: {os.path.getsize(output_path)} bytes")

Actual outcome

/usr/local/pip/matplotlib/mpl-data/fonts/ttf/cmtt10.ttf In FT2Font: Can not load face (unknown file format; error code 0x2)

Matplotlib backend: agg
Selected font: /usr/local/pip/matplotlib/mpl-data/fonts/ttf/DejaVuSans.ttf
Font exists: True
Traceback (most recent call last):
File "/u/sri/tlh_analyse_try/test_matplotlib_uss.py", line 23, in
File "/usr/local/pip/matplotlib/pyplot.py", line 1134, in savefig
res = fig.savefig(*args, **kwargs) # type: ignore[func-returns-value]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/figure.py", line 3390, in savefig
self.canvas.print_figure(fname, **kwargs)
File "/usr/local/pip/matplotlib/backend_bases.py", line 2193, in print_figure
result = print_method(
^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/backend_bases.py", line 2043, in
print_method = functools.wraps(meth)(lambda *args, **kwargs: meth(
^^^^^
File "/usr/local/pip/matplotlib/backends/backend_agg.py", line 497, in print_png
self._print_pil(filename_or_obj, "png", pil_kwargs, metadata)
File "/usr/local/pip/matplotlib/backends/backend_agg.py", line 445, in _print_pil
FigureCanvasAgg.draw(self)
File "/usr/local/pip/matplotlib/backends/backend_agg.py", line 388, in draw
self.figure.draw(self.renderer)
File "/usr/local/pip/matplotlib/artist.py", line 95, in draw_wrapper
result = draw(artist, renderer, *args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/artist.py", line 72, in draw_wrapper
return draw(artist, renderer)
^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/figure.py", line 3154, in draw
mimage._draw_list_compositing_images(
File "/usr/local/pip/matplotlib/image.py", line 132, in _draw_list_compositing_images
a.draw(renderer)
File "/usr/local/pip/matplotlib/artist.py", line 72, in draw_wrapper
return draw(artist, renderer)
^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/axes/_base.py", line 3034, in draw
self._update_title_position(renderer)
File "/usr/local/pip/matplotlib/axes/_base.py", line 2978, in _update_title_position
ax.yaxis.get_tightbbox(renderer) # update offsetText
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/axis.py", line 1336, in get_tightbbox
self._update_label_position(renderer)
File "/usr/local/pip/matplotlib/axis.py", line 2609, in _update_label_position
bboxes, bboxes2 = self._get_tick_boxes_siblings(renderer=renderer)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/axis.py", line 2161, in _get_tick_boxes_siblings
tlb, tlb2 = axis._get_ticklabel_bboxes(ticks_to_draw, renderer)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/axis.py", line 1315, in _get_ticklabel_bboxes
return ([tick.label1.get_window_extent(renderer)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/text.py", line 956, in get_window_extent
bbox, info, descent = self._get_layout(self._renderer)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/text.py", line 373, in _get_layout
_, lp_h, lp_d = _get_text_metrics_with_cache(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/text.py", line 69, in _get_text_metrics_with_cache
return _get_text_metrics_with_cache_impl(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/text.py", line 77, in _get_text_metrics_with_cache_impl
return renderer_ref().get_text_width_height_descent(text, fontprop, ismath)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/backends/backend_agg.py", line 220, in get_text_width_height_descent
font = self._prepare_font(prop)
^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/backends/backend_agg.py", line 254, in _prepare_font
font = get_font(_fontManager._find_fonts_by_props(font_prop))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/pip/matplotlib/font_manager.py", line 1554, in get_font
return _get_font(
^^^^^^^^^^
File "/usr/local/pip/matplotlib/font_manager.py", line 1496, in _get_font
return ft2font.FT2Font(
^^^^^^^^^^^^^^^^
RuntimeError: In FT2Font: Can not load face (unknown file format; error code 0x2)

Expected outcome

should label the plot

Additional information

I created a smoke-test after it did not work in a larger environment that works in ubuntu, but not under USS (z/OS)

Operating system

IBM USS that is z/OS

Matplotlib Version

3.8.4

Matplotlib Backend

No response

Python version

3.12

Jupyter version

No response

Installation

pip