So, you have the Arduino connected to the RPi by USB port, correct? And you are running the Arduino sketch on the RPi and getting data 9600 baud or similar, correct?
I'd use Processing (https://processing.org/
) for the real-time plotting. There are plenty of tutorials showing how to do this.
I haven't done it on a RPi, but I would expect the RPi 2 would have enough grunt.............