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Plot a PNG using matplotlib in a web request, using Flask.
"Plot a PNG using matplotlib in a web request, using Flask."
# Install dependencies, preferably in a virtualenv:
# pip install flask matplotlib
# Run the development server:
# python
# Go to http://localhost:5000/plot.png and see a plot of random data.
import random
import StringIO
from flask import Flask, make_response
from matplotlib.backends.backend_agg import FigureCanvasAgg as FigureCanvas
from matplotlib.figure import Figure
app = Flask(__name__)
def plot():
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)
canvas = FigureCanvas(fig)
output = StringIO.StringIO()
response = make_response(output.getvalue())
response.mimetype = 'image/png'
return response
if __name__ == '__main__':

sburns commented Apr 3, 2014

exactly what I needed, thank you.

i like it, thx!!!


mapfumo commented Sep 6, 2015

Awesome. Thank you.

smargs commented Oct 29, 2015

Hi, thank you for the code. I ran it and got this msg '* Restarting with stat'. So i installed watchdog suggested in this post and now I get this '* '*Restarting with windowsapi reloader'. And the localhost page doesn't work (keeps loading). Would you know how to make this work ? Thank you.

Is there a way to embed this into a flask template? Right now i'm passing the response variable into an html template using the render_template function, but the image is not showing up; instead, only the response string is rendering. Thank you in advance!

thank you for alternative way to show plotted graph !

Thanks a lot! This was super useful :)

I am using python3.5 and ran the code with slight modification:

  1. Instead of import StringIO, I used import io
  2. Instead of output = StringIO.StringIO(), I used output = io.BytesIO()


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