Graph module throwing error in ubuntu 18.04.1

[jdwillow]

MANUAL> Adding graph: ['VFR_HUD.alt']

/usr/local/lib/python2.7/dist-packages/MAVProxy/modules/lib/live_graph.py:55: UserWarning:
This call to matplotlib.use() has no effect because the backend has already
been chosen; matplotlib.use() must be called before pylab, matplotlib.pyplot,
or matplotlib.backends is imported for the first time.

The backend was originally set to u'TkAgg' by the following code:
File "/usr/lib/python2.7/threading.py", line 774, in __bootstrap
self.__bootstrap_inner()
File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
self.run()
File "/usr/lib/python2.7/threading.py", line 754, in run
self.__target(*self.__args, **self.__kwargs)
File "/usr/local/bin/mavproxy.py", line 796, in main_loop
process_stdin(c)
File "/usr/local/bin/mavproxy.py", line 521, in process_stdin
fn(args[1:])
File "/usr/local/lib/python2.7/dist-packages/MAVProxy/modules/mavproxy_graph.py", line 45, in cmd_graph
self.graphs.append(Graph(self, args[:]))
File "/usr/local/lib/python2.7/dist-packages/MAVProxy/modules/mavproxy_graph.py", line 90, in init
title=self.fields[0])
File "/usr/local/lib/python2.7/dist-packages/MAVProxy/modules/lib/live_graph.py", line 44, in init
self.child.start()
File "/usr/lib/python2.7/multiprocessing/process.py", line 130, in start
self._popen = Popen(self)
File "/usr/lib/python2.7/multiprocessing/forking.py", line 126, in init
code = process_obj._bootstrap()
File "/usr/lib/python2.7/multiprocessing/process.py", line 267, in _bootstrap
self.run()
File "/usr/lib/python2.7/multiprocessing/process.py", line 114, in run
self._target(*self._args, **self._kwargs)
File "/usr/local/lib/python2.7/dist-packages/MAVProxy/modules/lib/live_graph.py", line 53, in child_task
from live_graph_ui import GraphFrame
File "/usr/local/lib/python2.7/dist-packages/MAVProxy/modules/lib/live_graph_ui.py", line 3, in
import numpy, pylab
File "/usr/lib/python2.7/dist-packages/pylab.py", line 1, in
from matplotlib.pylab import *
File "/usr/lib/python2.7/dist-packages/matplotlib/pylab.py", line 257, in
from matplotlib import cbook, mlab, pyplot as plt
File "/usr/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 72, in
from matplotlib.backends import pylab_setup
File "/usr/lib/python2.7/dist-packages/matplotlib/backends/init.py", line 14, in
line for line in traceback.format_stack()

matplotlib.use('WXAgg')