diff --git a/optuna/visualization/_parallel_coordinate.py b/optuna/visualization/_parallel_coordinate.py
index 3c0b97a450..aa901a7dad 100644
--- a/optuna/visualization/_parallel_coordinate.py
+++ b/optuna/visualization/_parallel_coordinate.py
@@ -300,4 +300,4 @@ def _get_dims_from_info(info: _ParallelCoordinateInfo) -> list[dict[str, Any]]:
 
 
 def _truncate_label(label: str) -> str:
-    return label if len(label) < 20 else "{}...".format(label[:17])
+    return label if len(label) < 20 else label[:17] + "..."