Expected behavior

The range of objective in optuna.visualization.matplotlib.plot_contour should have a similar range to optuna.visualization.plot_contour.

Environment

• Optuna version: 2.7.0
• Python version: 3.8.8
• OS: Mac
• (Optional) Other libraries and their versions:
  • matplotlib: 3.3.4
  • Plotly: 4.11.0

Error messages, stack traces, or logs

The matplotlib range is from -60 000 to 360 000 as in the following figure.

On the other hand, Plotly's range is from 0 to 9 000. That is supposed to be true.

Steps to reproduce

1. Please run a following example code blocks

Reproducible examples (optional)

import optuna


def objective(trial):
    x = trial.suggest_uniform('x', -100, 100)
    y = trial.suggest_categorical('y', list(range(-10, 10)))
    z = trial.suggest_uniform('z', -100, 100)
    return x ** 2 + y - z

study = optuna.create_study(sampler=optuna.samplers.TPESampler(seed=7))
study.optimize(objective, n_trials=200)

optuna.visualization.matplotlib.plot_contour(study, params=['y', 'z'])
optuna.visualization.plot_contour(study, params=['y', 'z']) # plotly