docs: def y_label(self, y_label=None):
          """Returns or sets (if a value is provided) the chart's y-axis label.
  
          :param str y_label: If given, the chart's y_label will be set to this.
          :rtype: ``str``"""
  
          if y_label is None:
              return self._y_label
          else:
              if not isinstance(y_label, str):
                  raise TypeError("y_label must be str, not '%s'" % str(y_label))
              self._y_label = y_label