title: how to override a method from an inherited class
body: <p>So I use core-plot and I have to override a method in CPTLayer for CPTAxisLabel, but not for all labels, so I made an CustomCPTAxisLabel class to do that. 
  The problem is that I have to override renderAsVectorInContext witch is a method from CPTLayer, 
  CPTAxisLabel inherits CPTLayer and CustomCPTAxisLabel inherits CPTAxisLabel (something like this : CPTLayer->CPTAxisLabel->CustomCPTAxisLabel). The question is how can I override a method from CPTLayer in CustomCPTAxisLabel ?</p>