Add message about stopping the server

Author: mdboom

lib/matplotlib/backends/backend_webagg.py

@@ -484,6 +484,10 @@ def start(cls):
         if cls.started:
             return
 
-        tornado.ioloop.IOLoop.instance().start()
+        print("Press Ctrl+C to stop server")
+        try:
+            tornado.ioloop.IOLoop.instance().start()
+        except KeyboardInterrupt:
+            print("Server stopped")
 
         cls.started = True