I wonder if anyone ever posted an example project that uses Core Plot?
I would suggest the best website for iOS development tutorial. Here is the simplest core data example what you are looking for, with great explanation and source code.
http://www.raywenderlich.com/13269/how-to-draw-graphs-with-core-plot-part-1
http://www.raywenderlich.com/13271/how-to-draw-graphs-with-core-plot-part-2
Hope this helps.