index.py

# pip install fastapi "uvicorn[standard]" numpy matplotlib cairocffi

# run with uvicorn index:app (--reload)

import matplotlib
import matplotlib.pyplot as plt
import numpy as np

import io
from io import BytesIO

from fastapi import FastAPI
from fastapi.responses import StreamingResponse

# brew install cairo OR apt install libcairo2-dev
# pip install cairocffi (or pycairo, if that floats your boat)
matplotlib.use('Cairo')

app = FastAPI()

@app.get("/")
def read_root():
    return {"Hello": "World"}

@app.get("/plot.png")
def get_plot():
  image_buffer = BytesIO()
  x = np.linspace(0, 2 * np.pi, 200)
  y = np.sin(x)
  fig, ax = plt.subplots()

  ax.plot(x, y)
  plt.savefig(image_buffer, format="png")
  plt.close()
  image_buffer.seek(0)
  return StreamingResponse(image_buffer, media_type="image/png")