Any programming language that can read from and write to the serial port can be used. Objective-C has this ability.
Creating the real-time graph is an exercise in graphics manipulation. I can't comment on how easy or difficult that would be in Objective-C, since I've never tried that. Given the amazing things that Mac and iPhone and iPad applications can do, graphics-wise, I don't think it is a difficult task.
what is the best way to learn this language?
Same as any other language. Practice, practice, practice. When you get pretty good, practice some more.