FIX: Webagg save_figure raise a warning instead of an error

Author: blink1073

lib/matplotlib/backends/backend_webagg_core.py

@@ -19,6 +19,7 @@
 import json
 import os
 import time
+import warnings
 
 import numpy as np
 
@@ -318,6 +319,10 @@ def release_zoom(self, event):
         self.canvas.send_event(
             "rubberband", x0=-1, y0=-1, x1=-1, y1=-1)
 
+    def save_figure(self, *args):
+        """Save the current figure"""
+        warnings.warn('"Save figure" not implemented in Web Backend')
+
 
 class FigureManagerWebAgg(backend_bases.FigureManagerBase):
     ToolbarCls = NavigationToolbar2WebAgg