Code base could do with review and refactoring
Memory allocations, coding style, use of idioms, keeping up with Rust changes, module structure, visibility, unit tests. Since Subplot is my first big Rust program, it's starting to show layers of code where I've learnt or not learnt aspects of Rust development.
This should all be cleaned up by going though the code base with a critical eye and refactor it to be more uniform and generally better.