I kept getting the following linker error when i try using Core Plot in my project...
Undefined symbols for architecture i386:
\"_OBJC_CLASS_$_CPXYGraph\", ref
Add coreplotHeaders library from zip file downloaded from google code and also add libCoreplot-CocoaTouch.a to your project template.
NB:when adding don't create folder references for folders added.
I got the same error as you and my problem was that I had not added the QuartzCore framework (select your project target, expand "Link binaries with libraries", click '+' to select QuartzCore.framework).
Is this in xcode 4? You might want to make sure that you have the latest version of core plot. Xcode 4 caused a problem for me until I downloaded the latest version of core-plot and removed the previous directory.
I open the touch project and rebuilt then the linked was OK again!