Hi,
you'd proceed as follow.
You need two programs.
A first program uploaded to the arduino, which will send messages (for instance a temperature value) to the serial usb.
A second program which will run on the Pc, written in processing, to read that values and do a graph.
Arduino comes with some basic examples for communicating with Processing (in Arduino IDE, look at menu Examples > Communication).
A total different option is to drive Arduino via Processing, look at
http://www.arduino.cc/playground/Interfacing/Processing
But as far as I know, Processing uses Java, and arduino uses C++, so the two languages are a bit different.