Hmm ... I think I've got some linking/import issues, although the framework is found.
>> framework 'cocoa'
=> true
>> framework 'coreplot'
=> true
>> graph = CPXYGraph.alloc.initWithFrame(CGRectMake(0,0,0,0))
=> #<CPXYGraph:0x20054cb00>
>> plotSpace = graph.defaultPlotSpace;
=> #<CPXYPlotSpace:0x20051d360>
>> axisSet = graph.axisSet
=> #<CPXYAxisSet:0x2005229c0>
>>
So good, so far, but...
>> x = 4.2
=> 4.2
>> x.class
=> Float
>> z = CPDecimalFromFloat(x)
NoMethodError: undefined method `CPDecimalFromFloat' for main:TopLevel
>> theme = CPTheme.themeNamed(kCPDarkGradientTheme)
NameError: undefined local variable or method `kCPDarkGradientTheme' for main:TopLevel
>> CPTheme.method_defined?(:themeNamed)
=> false