[Tutor] What is wrong with my Python program that causes it to run but not give results?

[Ed Troy]

I found a very useful Python program for calculating the Spice 
parameters for a LED. I have Python3 and the various modules loaded onto 
my Ubuntu machine. I created the diode IV curve data as per the article, 
but I can't seem to get it to run. My python filename is 
LED_model_utf8.py  The text file with the LED IV data is LED_IV.txt.

When I type python LED_model_utf8.py LED_IV.txt, I get an error message:
edward at ubuntu:~$ python LED_model_utf8.py LED_IV.txt
Traceback (most recent call last):
   File "LED_model_utf8.py", line 4, in <module>
     import matplotlib.pyplot as plt
ImportError: No module named matplotlib.pyplot


If I type python3 LED_model_utf8.py LED_IV.txt, there is a pause and 
then I am back to the $ prompt. So, it seems like the program is 
running, but I am not getting any results like graphs and Spice 
parameters. Clearly, something is wrong, although the program "seems" to 
run correctly without errors. It just does not "do" anything.

The page that contains the file and a description of how it works is:
https://leicesterraspberrypi.wordpress.com/projects/modelling-a-diode-for-use-in-spice-simulations/

I can attach or list out my python file and the data file if needed.
Ed