code: def _format(self, serie, i):
          
          value = serie.values[i]
          metadata = serie.metadata.get(i)
  
          kwargs = {'chart': self, 'serie': serie, 'index': i}
          formatter = ((metadata and metadata.get('formatter'))
                       or serie.formatter or self.formatter
                       or self._value_format)
          kwargs = filter_kwargs(formatter, kwargs)
          return formatter(value, **kwargs)
docs: Format the nth value for the serie