Xcode 5.1 doesn't require projects to use ARC, it just finally drops support for garbage collection. Core Plot was never a strictly garbage-collected framework, but it did support building for garbage collection.
You can click on the "Not Now" option when presented with that dialog on opening the Core Plot project, then go to the project build settings. Blank out or remove the GCC_ENABLE_OBJC_GC setting under the user settings at the bottom of the project and the project will build just fine as a manually reference counted target. That's perfectly usable within an ARC or non-ARC application.