[Tutor] Using Matplotlib - program still running when graph window is closed

[AG]

How does one ensure that once a graph has been produced by Matplotlib 
and that graph has been closed by the user that the program itself stops?

What I am currently getting is that when I close the graph pop-up window 
and then close IDLE, I get a message that the program is still running 
and am I sure that I want to stop it.  Yes, I am sure, but I don't want 
to have to keep killing the IDLE interpreter window in order to do so, 
but if I don't, then I am seemingly unable to produce another graph 
pop-up window.

How do I control this from within the script itself?

TIA

AG