wfastcgi 500 error in flask app when trying to plot

Edit: Originally thought this was IIS issue, but looks like it's FastCgiModule causing the issue.

To be clear, the WebApp itself works, it only fails when I choose "Image Test" from the dropdown (see code below) menu on the site. Specifically, it fails on the df.plot line, and I can't figure out why.

from flask import Flask, request
import pandas as pd

app = Flask(__name__)

html = '''
<h1>Discovery Capital</h1>
<h2>Report Generator</h2>
<p>
<form action="/submitted" method="post">
  <label for="reports">Choose a Report:</label>
  <select id="reports" name="reports">
    <option value="img_test">Image Test</option>
  </select>
  <input type="submit">
</form>
'''

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

@app.route("/submitted", methods=['POST'])
def show():
    select = request.form.get("reports")

    if select == 'img_test':
        df = pd.DataFrame([1,2,3,4,5])
        df.plot()
        plt.close()
        result = "Success"

    else:
        result = "Not Available"

    return result

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

Appreciate the help!

Edit: Here's the log from failed request tracing:

from wfastcgi 500 error in flask app when trying to plot