Best way to manually control a core xy plotter.

Hello! I am working on a pretty standard coreXY plotter but am running into some road blocks on the firmware/code side of things. I want the plotter to be able to take constant inputs from an external sensor such as a webcam eye tracker to get an X/Y coordinate to draw.

Im using an Arduino Mega with the RAMPS interface. I was going to use a program like MAX MSP because I've used it to send data directly to Arduino boards in the past. A lot of the firmware I've looked at needs Gcode inputs which I don't know much of just yet. Any pointers or guidance would be really helpful!