Add support for an environment variable that disables pop-up graphs

[trevorhardy]

For purposes of automation and testing (particularly as Github Action), it would be useful to have a test for an environment variable that could be added to disable the pop-up results graphs and just save the output image to file.

[trevorhardy]

I didn't notice that the example runners already had a faux backend defined that prevents the graphs from displaying. So I'm guessing we don't need to add that functionality.

We still need some kind of checks to make sure the results produced in the automated test are correct.