fix ranking (lm-sys#3385)

Author: infwinston

fastchat/serve/monitor/monitor.py

@@ -303,7 +303,9 @@ def get_arena_table(arena_df, model_table_df, arena_subset_df=None):
         by=["final_ranking", "rating"], ascending=[True, False]
     )
     arena_df["final_ranking"] = recompute_final_ranking(arena_df)
-    arena_df = arena_df.sort_values(by=["final_ranking"], ascending=True)
+    arena_df = arena_df.sort_values(
+        by=["final_ranking", "rating"], ascending=[True, False]
+    )
 
     # sort by rating
     if arena_subset_df is not None: