Prevent test suite from generating matplotlib figures using X11-based backend

[fsaad]

Running the test suite using ./check.sh -s -vvv causes matplotlib figures to pop up, which need to be closed manually for the test to proceed.

• [ ] Determine which code is producing the figures.
• [ ] Use the 'agg' backend in those modules.

[axch]

I don't think the popping up is a function of the backend so much as something or other calling show() instead of writing figures to files. However, 'agg' is a good idea anyway, because it allows running tests that write figures to files even if the machine has no display connected.

[fsaad]

Sounds right, although with the agg backend show() has no blocking effect as far as I know.

[axch]

You may be right. I actually don't remember ever drawing a figure in agg and trying to show() it.