The way I would like to work is to have the repl open on the left hand side of the screen and code editor (like Sublime text, I'm sure many use Vim) on the right hand side.

I would run the code in repl (just using up-arrow and enter), get some plot, modify the code in the editor, save it, and rerun it repl.

Repl is used for connecting inputs to the program, not for editing. Often times you want to develop some small program that is part of a bigger program. So you run the big program up to the point it calls the small program. Or you run with some mock data. Then you have all your inputs for the small program, so you can start iterating it. Run - observe - edit - run - observe - edit.

In Matlab this worked because it detected if the file had changed, and reloaded it. You didn't specifically have to call load("my_function.m") before every call to it. This means you can just lift code from the big program and iterate on it in repl.