There already is a function, this is what’s being used for the progress bar in the game already.
I now have a health bar and an MP bar working, with one issue: I don’t know how to change the image/colour being used for the fill, which is why the MP bar uses green when it should be purple.
The compound bars might prove more difficult to sort out. The actual value in each case is easy, but at the moment not all of them have maximum values. ATP, for instance, doesn’t have a max. The rest of them share the same max storage space in the form of vacuoles.