rasterizing the heatmap layer into a tiny (blurry) bitmap when writing PDF

I noticed when we (cc @janosh) try to save the plotly generated heatmap with kaleido, the embedded heatmap in PDF became a tiny (and thus blurry) bitmap:

data.csv

repro.pdf

plotly=6.5.0
kaleido=1.2.0

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# /// script
# dependencies = ["pandas", "numpy", "plotly", "kaleido"]
# ///


from __future__ import annotations

from pathlib import Path
from importlib.metadata import version


import numpy as np
import pandas as pd
import plotly.graph_objects as go


print(f"plotly={version('plotly')}")
print(f"kaleido={version('kaleido')}")


def load_csv(csv_path: Path) -> pd.DataFrame:
    df = pd.read_csv(csv_path, index_col=0)
    df.index = pd.to_datetime(df.index.astype(str), format="%Y-%m", errors="raise")
    df.index.name = "time"
    return df


def prepare_heatmap_data(df_git: pd.DataFrame) -> tuple[pd.DataFrame, pd.DataFrame]:
    df_binned = df_git.resample("6ME").sum().rename_axis("time_binned")

    heatmap_data = df_binned.T
    heatmap_data.columns = heatmap_data.columns.to_series().dt.strftime("%Y-%m")

    log_data = np.log10(heatmap_data.replace(0, np.nan))
    return heatmap_data, log_data


def main() -> None:
    df_git = load_csv(Path("data.csv"))
    heatmap_data, log_data = prepare_heatmap_data(df_git)

    fig = go.Figure()
    fig.add_heatmap(
        z=log_data.to_numpy(),
        x=log_data.columns.to_list(),
        y=log_data.index,
        customdata=heatmap_data.to_numpy(),
    )
    fig.write_image("repro.pdf")


if __name__ == "__main__":
    main()

> pdfimages -list repro.pdf
page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
   1     0 image      29    13  icc     3   8  image  no         5  0     6     4 1035B  92%
   1     1 smask      29    13  gray    1   8  image  no         5  0     6     4   48B  13%

Viewing that same PDF in macos preview is blurry, but on macos Chrome seems fine:

Start by running the provided Python reproduction with plotly 6.5.0 and kaleido 1.2.0, then inspect repro.pdf with pdfimages as shown. Trace the PDF export path for the heatmap and verify that the generated PDF no longer embeds the heatmap as a 29×13 bitmap or appears blurry in macOS Preview.