WebAgg: reuse already running Tornado ioloop

    If a Tornado ioloop is already running before WebAggApplication.start() is called,
    it should be re-used instead of trying to start it again. This allows one to
    use WebAgg in his/her external Tornado application.

Author: vmarkovtsev

lib/matplotlib/backends/backend_webagg.py

@@ -369,11 +369,14 @@ def start(cls):
         if cls.started:
             return
 
-        print("Press Ctrl+C to stop server")
-        try:
-            tornado.ioloop.IOLoop.instance().start()
-        except KeyboardInterrupt:
-            print("Server stopped")
+        if not tornado.ioloop.IOLoop.instance().running():
+            print("Press Ctrl+C to stop server")
+            try:
+                tornado.ioloop.IOLoop.instance().start()
+            except KeyboardInterrupt:
+                print("Server stopped")
+        else:
+            print("Server is running in an existing Tornado IOLoop")
 
         cls.started = True