Shows how to use flask and matplotlib together.

flask_matplotlib.py

""" Shows how to use flask and matplotlib together.

Shows SVG, and png.
The SVG is easier to style with CSS, and hook JS events to in browser.

python3 -m venv venv
. ./venv/bin/activate
pip install flask matplotlib
python flask_matplotlib.py
"""
import io
import random
from flask import Flask, Response, request
from matplotlib.backends.backend_agg import FigureCanvasAgg
from matplotlib.backends.backend_svg import FigureCanvasSVG


from matplotlib.figure import Figure


app = Flask(__name__)


@app.route("/")
def index():
    """ Returns html with the img tag for your plot.
    """
    num_x_points = int(request.args.get("num_x_points", 50))
    # in a real app you probably want to use a flask template.
    return f"""
    <h1>Flask and matplotlib</h1>

    <h2>Random data with num_x_points={num_x_points}</h2>

    <form method=get action="/">
      <input name="num_x_points" type=number value="{num_x_points}" />
      <input type=submit value="update graph">
    </form>

    <h3>Plot as a png</h3>
    <img src="/matplot-as-image-{num_x_points}.png"
         alt="random points as png"
         height="200"
    >

    <h3>Plot as a SVG</h3>
    <img src="/matplot-as-image-{num_x_points}.svg"
         alt="random points as svg"
         height="200"
    >

    """
    # from flask import render_template
    # return render_template("yourtemplate.html", num_x_points=num_x_points)


@app.route("/matplot-as-image-<int:num_x_points>.png")
def plot_png(num_x_points=50):
    """ renders the plot on the fly.
    """
    fig = Figure()
    axis = fig.add_subplot(1, 1, 1)
    x_points = range(num_x_points)
    axis.plot(x_points, [random.randint(1, 30) for x in x_points])

    output = io.BytesIO()
    FigureCanvasAgg(fig).print_png(output)
    return Response(output.getvalue(), mimetype="image/png")


@app.route("/matplot-as-image-<int:num_x_points>.svg")
def plot_svg(num_x_points=50):
    """ renders the plot on the fly.
    """
    fig = Figure()
    axis = fig.add_subplot(1, 1, 1)
    x_points = range(num_x_points)
    axis.plot(x_points, [random.randint(1, 30) for x in x_points])

    output = io.BytesIO()
    FigureCanvasSVG(fig).print_svg(output)
    return Response(output.getvalue(), mimetype="image/svg+xml")


if __name__ == "__main__":
    import webbrowser

    webbrowser.open("http://127.0.0.1:5000/")
    app.run(debug=True)