energy_model: set matplotlib backent to AGG

Matplotlib defautls to the GTK backend. This can cause problems when
running in a headless session (e.g. over SSH). Since energy_model
istrument generates PNG plots, rather than rendering directly to UI, it
doesn't actually need GTK; set backend to AGG so that energy_model works
in headless environments.

wlauto/instrumentation/energy_model/__init__.py

@@ -11,6 +11,8 @@ from collections import Counter, namedtuple
 try:
     import jinja2
     import pandas as pd
+    import matplotlib
+    matplotlib.use('AGG')
     import matplotlib.pyplot as plt
     import numpy as np
     import_error = None