调用Python的PHP在没有安装库的情况下工作，但在具有MatPlotLib等库的浏览器上失败

I have a PHP code that calls a Python code in the back end. As long as there are no libraries involved (I use matplotlib in this case), the output of the python is displayed in the web-browser. However, when I import matplotlib, the browser just goes blank. I know this is a permission issue, but cannot recall how to set up the permissions of the libraries correctly.

I am running on AWS ubuntu, with PHP and Python3

Here is the PHP code

<?php
error_reporting(E_ALL);

ini_set('display_errors', 1);

$output = shell_exec('/home/ubuntu/plotExample.py');

echo $output;

?>

Here is the python example:

#!/usr/bin/python3
import sys

from matplotlib.pyplot import figure

print ("Hello World")

What works (prints "Hello World"):

php /var/www/html/phptest.php  

Same php code on browser gives a blank page.

Now, if I comment out "from matplotlib.pyplot import figure", the Hello World shows up in the browser