Make inline the default matplotlib backend

uses MPLBACKEND env (matplotlib > 1.5).

Trigger extra IPython inline setup when inline backend is loaded.
We can't call the simpler `ip.enable_matplotlib()` here because
this will be triggered during pyplot import,
and enable_matplotlib() imports pyplot which isn't ready yet.

ipykernel/kernelapp.py

@@ -368,6 +368,13 @@ def init_kernel(self):
     def init_gui_pylab(self):
         """Enable GUI event loop integration, taking pylab into account."""
 
+        # Register inline backend as default
+        # this is higher priority than matplotlibrc,
+        # but lower priority than anything else (mpl.use() for instance).
+        # This only affects matplotlib >= 1.5
+        if not os.environ.get('MPLBACKEND'):
+            os.environ['MPLBACKEND'] = 'module://ipykernel.pylab.backend_inline'
+
         # Provide a wrapper for :meth:`InteractiveShellApp.init_gui_pylab`
         # to ensure that any exception is printed straight to stderr.
         # Normally _showtraceback associates the reply with an execution,

ipykernel/pylab/backend_inline.py

@@ -143,3 +143,14 @@ def flush_figures():
 # figurecanvas. This is set here to a Agg canvas
 # See https://github.com/matplotlib/matplotlib/pull/1125
 FigureCanvas = FigureCanvasAgg
+
+def _enable_matplotlib_integration():
+    """Enable extra IPython matplotlib integration when we are loaded as the matplotlib backend."""
+    from matplotlib import get_backend
+    from IPython.core.pylabtools import configure_inline_support
+    ip = get_ipython()
+    backend = get_backend()
+    if ip and backend == 'module://%s' % __name__:
+        configure_inline_support(ip, backend)
+
+_enable_matplotlib_integration()