Safely import pyplot if a GUI framework is already running.

`configure_subplots` now needs to import `pyplot` (because that's
how we can spin up a new figure as needed), but when relying on
embedding we could well reach that point with an already running
interactive framework, pyplot never imported, and an incompatible
rcParams["backend"] (which had not been read yet).  For example, run
`examples/user_interfaces/embedding_in_tk_sgskip.py` with a non-tk
backend set in your matplotlibrc: clicking on the the configure_subplots
button triggers a "cannot load backend 'foo' which requires the 'foo'
interactive framework, as 'bar' is currently running".

Instead, use a helper to safely import pyplot when another interactive
framework may already be running.  The list of frameworks that
`_get_running_interactive_framework` returns is fixed, so we can just
hardcode the mapping.  (We are going to ignore the user-set
`rcParams["backend"]` anyways.)

Author: anntzer

lib/matplotlib/backend_bases.py

@@ -85,6 +85,30 @@
 }
 
 
+def _safe_pyplot_import():
+    """
+    Import and return ``pyplot``, correctly setting the backend if one is
+    already forced.
+    """
+    try:
+        import matplotlib.pyplot as plt
+    except ImportError:  # Likely due to a framework mismatch.
+        current_framework = cbook._get_running_interactive_framework()
+        if current_framework is None:
+            raise  # No, something else went wrong, likely with the install...
+        backend_mapping = {'qt5': 'qt5agg',
+                           'qt4': 'qt4agg',
+                           'gtk3': 'gtk3agg',
+                           'wx': 'wxagg',
+                           'tk': 'tkagg',
+                           'macosx': 'macosx',
+                           'headless': 'agg'}
+        backend = backend_mapping[current_framework]
+        rcParams["backend"] = mpl.rcParamsOrig["backend"] = backend
+        import matplotlib.pyplot as plt  # Now this should succeed.
+    return plt
+
+
 def register_backend(format, backend, description=None):
     """
     Register a backend for saving to a given file format.
@@ -3269,7 +3293,7 @@ def _update_view(self):
         self.canvas.draw_idle()
 
     def configure_subplots(self, *args):
-        from matplotlib import pyplot as plt
+        plt = _safe_pyplot_import()
         tool = plt.subplot_tool(self.canvas.figure)
         tool.figure.canvas.manager.show()