scripts/graph-build-time: clarify backend selection

This instruction in the middle of 'import' lines looks very strange.

Also, it was not obvious to me what the 'Agg' backend is.

Both things are actually correct, but it took a while to find out why.
So clarify with a comment to save someone else's time.

[Peter: fix s/soe/some/ typo]
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Sascha Arthur <sascha.arthur@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/scripts/graph-build-time

@@ -52,7 +52,12 @@
 import matplotlib as mpl
 import numpy
 
+# Use the Agg backend (which produces a PNG output, see
+# http://matplotlib.org/faq/usage_faq.html#what-is-a-backend),
+# otherwise an incorrect backend is used on some host machines).
+# Note: matplotlib.use() must be called *before* matplotlib.pyplot.
 mpl.use('Agg')
+
 import matplotlib.pyplot as plt
 import matplotlib.font_manager as fm
 import csv