If you want the whole graph to be visible at the same time, you probably want one of our 2.8" TFT breakouts or shields: http://www.adafruit.com/products/376http://www.adafruit.com/products/335
We have software libraries that make using the display more or less a plug and play affair, plus a graphics library that has primitives for drawing lines.
We have 7" displays (and larger) but to make them work you'd need more hardware. The displays we sell use NTSC/PAL signals, and generating one of those from an Arduino is a pretty CPU and timing intensive business. The Raspberri Pi has composite NTSC/PAL/HDMI video built in, runs Linux so you'd have a wide array of Open Source graphing and graphics software at your disposal, and has a range of GPIO pins that can probably be programmed to do anything an Arduino can do.
It's also possible to establish a serial connection between a RasPi and an Arduino so the Arduino does low-level hardware management and the RasPi does high-level data processing and display.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.