Fix bug of pareto front graph

[c-bata]

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Reference Issues/PRs

Fixes #206

What does this implement/fix? Explain your changes.

Steps to reproduce:

$ python -c "import optuna; print(optuna.version.__version__)"
3.0.0b0
$ optuna-dashboard --version
0.6.3
$ cat example_failure.py
import optuna


def objective(trial):
    x = trial.suggest_uniform('x', -10, 10)
    y = trial.suggest_uniform('y', -10, 10)
    z = trial.suggest_uniform('z', -10, 10)
    return (x - 2) ** 2, (y - 2) ** 2, (z - 2) ** 2

if __name__ == "__main__":
    sampler = optuna.samplers.TPESampler(n_startup_trials=10, n_ei_candidates=24)
    for i in range(0, 10):
        study = optuna.create_study(storage="sqlite:///multiobjective-failure-db.sqlite3", study_name="issue-206", directions=['minimize','minimize','minimize'], load_if_exists=True)
        study.optimize(objective, n_trials=3)
$ python example_failure.py  # then press Ctrl-C immediately
$ optuna-dashboard sqlite:///multiobjective-failure-db.sqlite3

Before	After