If the study already exists, it will be simply loaded.
    

    search_space must be a string that can be evaluated to a dictionary to specify Optuna's distributions.

    Example:
        {"x": {"name": "FloatDistribution", "attributes": {"step": null, "low": -10.0, "high": 10.0, "log": false}}}
    

    The default sampler for single-objective optimization is TPESampler.
    The default sampler for multi-objective optimization is NSGAIISampler.
    GPSampler is a Gaussian process-based sampler suitable for low-dimensional numerical optimization problems.
    

    Args:
        metric_names:
            The list of metric name for each objective value.
            The length of metric_names list must be the same with the number of objectives.
    

    This feature can only be used for single-objective optimization. If your study is multi-objective, use best_trials instead.
    

    Args:
        target:
            An index to specify the value to display. To plot nth objective value, set this to n.
            Note that this is 0-indexed, i.e., to plot the first objective value, set this to 0.
            For single-objective optimization, None (auto) is recommended.
            For multi-objective optimization, this must be specified.
        target_name:
            Target's name to display on the axis label and the legend.
    

    Args:
        reference_point:
            A list of reference points to calculate the hypervolume.
    

    Args:
        target_names:
            Objective name list used as the axis titles. If :obj:`None` is specified,
            "Objective {objective_index}" is used instead. If ``targets`` is specified
            for a study that does not contain any completed trial,
            ``target_name`` must be specified.
        include_dominated_trials:
            A flag to include all dominated trial's objective values.
        targets:
            A list of indices to specify the objective values to display.
            Note that this is 0-indexed, i.e., to plot the first and second objective value, set this to [0, 1].
            If the number of objectives is neither 2 nor 3, ``targets`` must be specified.
            By default, all objectives are displayed.
    

    Args:
        params:
            Parameter list to visualize. The default is all parameters.
        target:
            An index to specify the value to display. To plot nth objective value, set this to n.
            Note that this is 0-indexed, i.e., to plot the first objective value, set this to 0.
        target_name:
            Target’s name to display on the color bar.
    

    Args:
        params:
            Parameter list to visualize. The default is all parameters.
        target:
            An index to specify the value to display. To plot nth objective value, set this to n.
            Note that this is 0-indexed, i.e., to plot the first objective value, set this to 0.
        target_name:
            Target’s name to display on the axis label and the legend.
    

    Args:
        params:
            Parameter list to visualize. The default is all parameters.
        target:
            An index to specify the value to display. To plot nth objective value, set this to n.
            Note that this is 0-indexed, i.e., to plot the first objective value, set this to 0.
        target_name:
            Target’s name to display on the axis label.
    

    Args:
        params:
            Parameter list to visualize. The default is all parameters.
        target:
            An index to specify the value to display. To plot nth objective value, set this to n.
            Note that this is 0-indexed, i.e., to plot the first objective value, set this to 0.
            By default, all objective will be plotted by setting target to None.
        target_name:
            Target’s name to display on the legend.
    

    Args:
        target:
            An index to specify the value to display. To plot nth objective value, set this to n.
            Note that this is 0-indexed, i.e., to plot the first objective value, set this to 0.
        target_name:
            Target’s name to display on the axis label.
    

    Args:
        params:
            Parameter list to visualize. The default is all parameters.
        target:
            An index to specify the value to display. To plot nth objective value, set this to n.
            Note that this is 0-indexed, i.e., to plot the first objective value, set this to 0.
        target_name:
            Target’s name to display on the color bar.