Example of rendering a matplotlib image directly to Flask view

flaskplotlib.py

from flask import Flask, make_response, render_template
app = Flask(__name__)

@app.route("/")
def index():
    render_template("index.html")

@app.route("/simple.png")
def simple():
    import datetime
    import StringIO
    import random

    from matplotlib.backends.backend_agg import FigureCanvasAgg as FigureCanvas
    from matplotlib.figure import Figure
    from matplotlib.dates import DateFormatter

    fig=Figure()
    ax=fig.add_subplot(111)
    x=[]
    y=[]
    now=datetime.datetime.now()
    delta=datetime.timedelta(days=1)
    for i in range(10):
        x.append(now)
        now+=delta
        y.append(random.randint(0, 1000))
    ax.plot_date(x, y, '-')
    ax.xaxis.set_major_formatter(DateFormatter('%Y-%m-%d'))
    fig.autofmt_xdate()
    canvas=FigureCanvas(fig)
    png_output = StringIO.StringIO()
    canvas.print_png(png_output)
    response=make_response(png_output.getvalue())
    response.headers['Content-Type'] = 'image/png'
    return response


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

index.html

{% extends "layout.html" %}
{% block body %}
  <h1>Render a Image Directly by matplotlib</h1>
  <img src="{{ url_for('simple') }}" alt="loading..." />
{% endblock %}

Demonstrate how to embedded the rendered image in other webpage other than in a single webpage.

I'm totally new to Web Programming, but it seems that this code is not going to work. I kept getting Internal Server Error from it.