use z_index to have edges below vertices

Author: behackl

manim/mobject/graph.py

@@ -254,15 +254,16 @@ def __init__(
                     edge_type(
                         self[u].get_center(),
                         self[v].get_center(),
+                        z_index=-1,
                         **self._edge_config[(u, v)],
                     ),
                 )
                 for (u, v) in edges
             ]
         )
 
-        self.add(*self.edges.values())
         self.add(*self.vertices.values())
+        self.add(*self.edges.values())
 
         for (u, v), edge in self.edges.items():