Best way to make an interactive graphic widget? (souped up kslider)

Hi all, I'm working on some music pedagogy tools and I want to add a much souped up klsider-like object that I can use to show key sequences with numerals drawn on the keyboard, or different key colours, etc. I've only done music-event and audio hacking in Max so far, can anyone tell what the right path for making an interactive GUI widget is? FWIW, I'm a professional developer so if the answer is to do it in C with OpenGL, that's fine. Just hoping someone who's make interactive graphic widgets can weigh in and prevent me spinning wheels trying the wrong approach.

Thanks!
Iain