docs: def CO_ratio(self,ifig,ixaxis):
          """
          plot surface C/O ratio in Figure ifig with x-axis quantity ixaxis
  
          Parameters
          ----------
          ifig : integer
              Figure number in which to plot
          ixaxis : string
              what quantity is to be on the x-axis, either 'time' or 'model'
              The default is 'model'
          """
  
          def C_O(model):
              surface_c12=model.get('surface_c12')
              surface_o16=model.get('surface_o16')
              CORatio=old_div((surface_c12*4.),(surface_o16*3.))
              return CORatio
  
          if ixaxis=='time':
              xax=self.get('star_age')
          elif ixaxis=='model':
              xax=self.get('model_number')
          else:
              raise IOError("ixaxis not recognised")
  
          pl.figure(ifig)
          pl.plot(xax,C_O(self))