added %matplotlib --list so backends are easily visible

IPython/core/magics/pylab.py

@@ -42,6 +42,8 b' class PylabMagics(Magics):'
     @skip_doctest
     @line_magic
     @magic_arguments.magic_arguments()
+    @magic_arguments.argument('-l', '--list', default=None, action='store_true',
+                              help='Show available matplotlib backends')
     @magic_gui_arg
     def matplotlib(self, line=''):
         """Set up matplotlib to work interactively.
@@ -84,9 +86,15 b' class PylabMagics(Magics):'
 
             In [3]: %matplotlib qt
         """
+
         args = magic_arguments.parse_argstring(self.matplotlib, line)
-        gui, backend = self.shell.enable_matplotlib(args.gui)
-        self._show_matplotlib_backend(args.gui, backend)
+
+        if args.list is not None:
+            backends_list = list(backends.keys())
+            print("Available matplotlib backends: %s" % backends_list)
+        else:
+            gui, backend = self.shell.enable_matplotlib(args.gui)
+            self._show_matplotlib_backend(args.gui, backend)
 
     @skip_doctest
     @line_magic