Update app.py

app.py

@@ -70,7 +70,7 @@ g = ig.Graph.Tree(7, 2)  # A binary tree with 7 nodes
 lay = g.layout('rt')  # Reingold-Tilford layout
 
 # Prepare node and edge traces for Plotly
-edge_trace = go.Scatter(
+edge_trace = [go.Scatter(
     x=[lay[edge[0]][0], lay[edge[1]][0], None],
     y=[lay[edge[0]][1], lay[edge[1]][1], None],
     line=dict(width=0.5, color='#888'),