scripts/graph-build-time: remove X dependency

[Thomas: tweak commit title.]

Signed-off-by: Sascha Arthur <sascha.arthur@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

support/scripts/graph-build-time

@@ -49,9 +49,10 @@
 #   * argparse (by default in Python 2.7, requires python-argparse if
 #     Python 2.6 is used)
 
-import matplotlib
+import matplotlib as mpl
 import numpy
 
+mpl.use('Agg')
 import matplotlib.pyplot as plt
 import matplotlib.font_manager as fm
 import csv