Unknown class CPTGraphHostingView in Interface Builder file.
2012-01-09 16:09:34.242 ChartArea[2595:207] -[UIView setHostedGraph:]: unrecognized selector sent to ins
I had the same problem and this worked for me:
Select your project from the navigator panel in Xcode and then highlight the Build Settings tab. Filter the settings or locate ‘Other Linker Flags’ from this list and then add -ObjC -all_load to these settings
(From here: http://www.johnwordsworth.com/2011/10/adding-charts-to-your-iphone-ipad-app-using-core-plot/)