Don't depend on django-tk or X-Windows server.

Change 42d20dc6ae8b7216b2dccf441cf78c52872ed3bd was in error.

We need to specify the matplotlib backend to use:

Matplotlib.use('Agg')

This was accidentally removed in
cd69810c0f105df5afd79966585d5bddd77a48a5.

Change-Id: Ibefeedb289c23e622275dc75502fc452c3714c07