Hiroki Takizawa

LGTM! `@parametrize_plot_timeline` seems to be not necessary for `test_get_timeline_plot_with_killed_running_trials` function, but it could be interpreted as preparation for future (e.g., support of another visualization library).

I have started working on this issue.

I think Optuna Dashboard (and user-defined tools) may expects `.log` to open a file, so I renamed with `journal_file_storage_jsonl.log` to avoid troubles.

The new apperance: 

Sorry for my late reply. I agree; I have modified the code.

Thank you for your PR! I think the recent comment and the current PR implementation looks generally reasonable. However, I would prefer not to use nested functions in the implementation...

@gen740 @not522 Could you review this PR?