Passing a matplotlib figure to HTML (flask)

You have to separate the HTML and the image into two different routes.

Your /images/<cropzonekey> route will just serve the page, and in the HTML content of that page there will be a reference to the second route, the one that serves the image.

The image is served in its own route from a memory file that you generate with savefig().

I obviously didn't test this, but I believe the following example will work as is or will get you pretty close to a working solution:

@app.route('/images/<cropzonekey>')
def images(cropzonekey):
    return render_template("images.html", title=cropzonekey)

@app.route('/fig/<cropzonekey>')
def fig(cropzonekey):
    fig = draw_polygons(cropzonekey)
    img = StringIO()
    fig.savefig(img)
    img.seek(0)
    return send_file(img, mimetype='image/png')

Your images.html template the becomes:

<html>
  <head>
    <title>{{ title }} - image</title>
  </head>
  <body>
    <img src="{{ url_for('fig', cropzonekey = title) }}" alt="Image Placeholder" height="100">
  </body>
</html>

Python 3

I went through a lot of trouble with errors like - Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'NSWindow drag regions should only be invalidated on the Main Thread!

For all those who want to use matplotlib with flask and render the graph on an html page in python 3, here you go -

In the __init__.py

import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
from flask import Flask, render_template
from io import BytesIO
import base64

    @app.route('/plot')
    def plot():
        img = BytesIO()
        y = [1,2,3,4,5]
        x = [0,2,1,3,4]

        plt.plot(x,y)

        plt.savefig(img, format='png')
        plt.close()
        img.seek(0)
        plot_url = base64.b64encode(img.getvalue()).decode('utf8')

        return render_template('plot.html', plot_url=plot_url)

In flaskr/templates/plot.html

<!doctype html>
<title>heatmap - </title>
<section>
  <h2>Heatmap</h2>
  <img src="data:image/png;base64, {{ plot_url }}">
</section>

Tags:

Python

Html

Image

Matplotlib

Flask

Recent Posts