From aa1381070cdf86c13b96df9db98d9d4ab14ff72c Mon Sep 17 00:00:00 2001
From: kAIto47802 <115693559+kAIto47802@users.noreply.github.com>
Date: Tue, 20 Aug 2024 16:30:26 +0900
Subject: [PATCH 1/2] Fix failure

---
 optuna/visualization/_contour.py                 | 6 +++---
 tests/visualization_tests/test_visualizations.py | 4 ----
 2 files changed, 3 insertions(+), 7 deletions(-)

diff --git a/optuna/visualization/_contour.py b/optuna/visualization/_contour.py
index a71607b32d..5e62c6b256 100644
--- a/optuna/visualization/_contour.py
+++ b/optuna/visualization/_contour.py
@@ -208,6 +208,9 @@ 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:
+        return go.Contour(), go.Scatter(), go.Scatter()
+
     feasible = _PlotValues([], [])
     infeasible = _PlotValues([], [])
 
@@ -227,9 +230,6 @@ def _get_contour_subplot(
 
     z_values[xys[:, 1], xys[:, 0]] = zs
 
-    if len(x_indices) < 2 or len(y_indices) < 2:
-        return go.Contour(), go.Scatter(), go.Scatter()
-
     contour = go.Contour(
         x=x_indices,
         y=y_indices,
diff --git a/tests/visualization_tests/test_visualizations.py b/tests/visualization_tests/test_visualizations.py
index 71cac17e77..a6e7439ce5 100644
--- a/tests/visualization_tests/test_visualizations.py
+++ b/tests/visualization_tests/test_visualizations.py
@@ -87,10 +87,6 @@ def test_visualizations_with_single_objectives(
     study = optuna.create_study(sampler=optuna.samplers.RandomSampler())
     study.optimize(objective_func, n_trials=20)
 
-    # TODO(c-bata): Fix a bug to remove `pytest.xfail`.
-    if plot_func is plot_contour and objective_func is objective_single_dynamic_with_categorical:
-        pytest.xfail("There is a bug that IndexError is raised in plot_contour")
-
     # TODO(c-bata): Fix a bug to remove `pytest.xfail`.
     if plot_func is matplotlib_plot_rank and objective_func is objective_single_none_categorical:
         pytest.xfail("There is a bug that TypeError is raised in matplotlib.plot_rank")

From 76e6c64f68a59f69d776f9304800cd42370e796d Mon Sep 17 00:00:00 2001
From: kAIto47802 <115693559+kAIto47802@users.noreply.github.com>
Date: Tue, 20 Aug 2024 16:43:12 +0900
Subject: [PATCH 2/2] Add warnings

---
 optuna/visualization/_contour.py | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/optuna/visualization/_contour.py b/optuna/visualization/_contour.py
index 5e62c6b256..44904c0425 100644
--- a/optuna/visualization/_contour.py
+++ b/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([], [])