code: def add_missing(self, aesthetics):
          
          # Keep only aesthetics that don't have scales
          aesthetics = set(aesthetics) - set(self.input())
  
          for ae in aesthetics:
              scale_name = 'scale_{}_continuous'.format(ae)
              scale_f = Registry[scale_name]
              self.append(scale_f())
docs: Add missing but required scales.
  
          Parameters
          ----------
          aesthetics : list | tuple
              Aesthetic names. Typically, ('x', 'y').