Add warnings

optuna/visualization/_contour.py

@@ -4,6 +4,7 @@
 from typing import Any
 from typing import Callable
 from typing import NamedTuple
+import warnings
 
 import numpy as np
 
@@ -208,7 +209,13 @@ def _get_contour_subplot(
     x_indices = info.xaxis.indices
     y_indices = info.yaxis.indices
 
-    if len(x_indices) < 2 or len(y_indices) < 2 or len(info.z_values) == 0:
+    if len(x_indices) < 2 or len(y_indices) < 2:
+        return go.Contour(), go.Scatter(), go.Scatter()
+    if len(info.z_values) == 0:
+        warnings.warn(
+            f"Contour plot will not be displayed because `{info.xaxis.name}` and "
+            f"`{info.yaxis.name}` cannot co-exist in `trial.params`."
+        )
         return go.Contour(), go.Scatter(), go.Scatter()
 
     feasible = _PlotValues([], [])