This issue proposes to implement the Matplotlib backend for the visualization functions.

Motivation

While the current visualization backend, Plotly, provides rich and interactive interfaces, the setup is sometimes hard especially when exporting figures as static ones. Specifically, the installation steps of Orca are complicated in some environments (see plotly/orca#150 for example) or may restrict the way of package management as plotly-orca favors Conda installation.

Providing another backend such as Matpotlib may mitigate this problem, where the library does not require extra setup to output static figures and is pre-installed in some popular platforms such as Anaconda.

Alternatives

Plotly has supported Kaleido backend for static export since Plotly 4.9. Thus, we could guide users to install this backend, although it restricts the Plotly version where Optuna supports >= 4.0.0 currently.