magfit: Use wxagg on Windows

Author: stephendade

MAVProxy/modules/lib/magfit.py

@@ -7,7 +7,7 @@
 from MAVProxy.modules.lib import wx_processguard
 from MAVProxy.modules.lib.wx_loader import wx
 
-import sys, time, os, math, copy
+import sys, time, os, math, copy, platform
 
 from pymavlink import mavutil
 from pymavlink import mavextra
@@ -18,6 +18,8 @@
 from MAVProxy.modules.lib.multiproc_util import MPDataLogChildTask
 
 import matplotlib
+if platform.system() == "Windows":
+    matplotlib.use('wxagg')
 import matplotlib.pyplot as pyplot
 import numpy
 import datetime