Try serving geopandas/matplotlib content with Flask.

app.py

"""
Try serving geopandas/matplotlib content with Flask. 

Run with:

    flask --app app run
"""

from flask import Flask, Response


app = Flask(__name__)


import io
import base64
import random
from matplotlib.figure import Figure

# https://github.com/matplotlib/matplotlib/blob/944b99a826a1b3509cbf7add49dccb14db9240fb/examples/user_interfaces/web_application_server_sgskip.py
@app.route("/nopyplot")
def nopyplot():
    # works
    # Generate the figure **without using pyplot**.
    fig = Figure()
    ax = fig.subplots()
    ax.plot([1, 2])
    # Save it to a temporary buffer.
    buf = io.BytesIO()
    fig.savefig(buf, format="png")
    # Embed the result in the html output.
    data = base64.b64encode(buf.getbuffer()).decode("ascii")
    return f"<img src='data:image/png;base64,{data}'/>"



import io
import random
from matplotlib.backends.backend_agg import FigureCanvasAgg
from matplotlib.figure import Figure


@app.route('/lineplot')
def mpl_plot_png():
    # works, too
    fig = Figure()
    axis = fig.add_subplot(1, 1, 1)
    xs = range(100)
    ys = [random.randint(1, 50) for x in xs]
    axis.plot(xs, ys)
    
    output = io.BytesIO()
    FigureCanvasAgg(fig).print_png(output)
    return Response(output.getvalue(), mimetype='image/png')


import geopandas

@app.route('/geoplot')
def gdf_plot_png():
    # crashes the process
    path = geopandas.datasets.get_path("nybb")
    df = geopandas.read_file(path)
    df_wm = df.to_crs(epsg=3857)  # Web Mercator
    # The following line crashes with:
    #   ... python3.10/site-packages/geopandas/plotting.py:673: UserWarning: 
    #   Starting a Matplotlib GUI outside of the main thread will likely fail.
    ax = df_wm.plot(figsize=(4, 4), alpha=0.5, edgecolor="k")
    fig = ax.figure
    
    output = io.BytesIO()
    FigureCanvasAgg(fig).print_png(output)
    return Response(output.getvalue(), mimetype="image/png")


if __name__ == "__main__":
   app.run(debug=True)