Standardizing Serif Fonts for Matplotlib on RHEL 9

This document explains how to configure Matplotlib on RHEL 9 so that PDF output uses Times-style Type 1 PostScript core fonts through the system-provided URW Base35 font package. This produces compact PDFs with consistent serif rendering (Nimbus Roman) across all users, without embedding large TrueType fonts.

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1. Check Whether Nimbus Roman Is Present

Run the following:

fc-list : family | grep -i "nimbus"

You should see entries such as:

Nimbus Roman
Nimbus Sans
Nimbus Mono PS

If Nimbus Roman appears, continue to Step 4.

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2. Install URW Base35 Fonts (if missing)

Install the font package:

sudo dnf install urw-base35-fonts

Verify the font files exist:

ls /usr/share/fonts/urw-base35 | grep -i nimbusroman

Look for files like:

NimbusRoman-Regular.otf
NimbusRoman-Italic.otf
NimbusRoman-Bold.otf
NimbusRoman-BoldItalic.otf

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3. Rebuild Fontconfig Cache

sudo fc-cache -f -v

This ensures new fonts are visible to applications using Fontconfig (including Matplotlib).

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4. Confirm Nimbus Roman via Fontconfig and Matplotlib

Check again via Fontconfig:

fc-list : family | grep -i nimbus

Then verify through Matplotlib:

python3 - <<EOF
import matplotlib.font_manager as fm
families = sorted({f.name for f in fm.fontManager.ttflist})
print([f for f in families if "Nimbus" in f])
EOF

You should see something like:

['Nimbus Roman', 'Nimbus Sans', 'Nimbus Mono PS']

Also check font resolution:

python3 - <<EOF
from matplotlib.font_manager import findfont
print(findfont("Nimbus Roman"))
EOF

Expected path example:

/usr/share/fonts/urw-base35/NimbusRoman-Regular.otf

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5. Clear Matplotlib Font Caches (per user)

Each user must run:

rm -f ~/.cache/matplotlib/fontlist-*.json
rm -rf ~/.cache/matplotlib/tex.cache

Caches regenerate automatically on next Matplotlib use.

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6. Required Matplotlib Runtime Settings

Add the following before creating any figures (or just use configure_fds_fonts() from fdsplotlib):

plt.rcParams.update({
    "pdf.use14corefonts": True,
    "text.usetex": False,

    "font.family": "serif",
    "font.serif": [
        "Nimbus Roman",
        "Times",
        "Times New Roman",
        "serif",
    ],

    "mathtext.fontset": "custom",
    "mathtext.rm":  "Nimbus Roman",
    "mathtext.it":  "Nimbus Roman:italic",
    "mathtext.bf":  "Nimbus Roman:bold",
    "mathtext.cal": "Nimbus Roman:italic",
    "mathtext.tt":  "Courier",
    "mathtext.default": "rm",

    "axes.unicode_minus": False,
    "pdf.compression": 9,
})

Results:

• serif text uses Nimbus Roman (Times-style)
• PDF uses core Type 1 fonts (Times/Helvetica/Courier)
• No embedded TTF fonts → tiny PDF sizes
• math text inherits Nimbus Roman styling

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7. Validate Correct Font Output

Create a test PDF:

python3 - <<EOF
import matplotlib.pyplot as plt
plt.text(0.5, 0.5, r"$\\nabla \\cdot \\mathbf{u} = 0$", fontsize=20)
plt.savefig("font_check.pdf")
EOF

List fonts used:

pdffonts font_check.pdf

Expected result:

Times-Roman      Type 1  (not embedded)
Times-Bold       Type 1  (not embedded)
Courier          Type 1  (not embedded)

Type 3 math paths may appear, which is normal.
No large embedded TTF entries should be present.

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8. Why Nimbus Roman

• Installed from official RHEL repos
• Metrics-compatible with Times Roman
• Legally redistributable
• Works correctly with pdf.use14corefonts=True
• Ensures consistent output for all cluster users
• Produces compact PDFs ideal for publications

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Summary Table

Step	Action
1	Confirm Nimbus Roman exists
2	Install urw-base35-fonts if needed
3	Run sudo fc-cache -f -v
4	Verify via Fontconfig and Matplotlib
5	Clear per-user Matplotlib cache
6	Apply the provided rcParams
7	Confirm fonts using pdffonts
8	Adopt Nimbus Roman as Times replacement

Once complete, all users will generate consistent, Times-style serif text in Matplotlib PDFs on RHEL 9 without embedding large TrueType fonts.