Off topic, but maybe some here has suggestions or pointers.
I’m looking for a tool, preferrably Android, that provides a sound generator that is X-Y controllable. Movement in the X direction changes the pitch (continuously) and movement in the Y direction changes the volume.
I would like to use this to mimic the sound of non-fretted strings like violin and cello.
Probably, but you’re missing an important part, which is onset.
MIDI is based on the notion that there is a chromatic scale for notes.
While there are MIDI parameters like pitch-bend, modulation, and tuning, it is not really useful for expressiveness of a slide-guitar, violin or fretless bass.
I would not really agree that MIDI has the idea of a chromatic scale for notes at its core. What is does have at its core is the notion of a keyboard-like mechanism, where notes begin and end at discrete points and retain the same pitch throughout. This is what breaks it as a model of playing non-fretted strings, or trombone, or vocals.