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``

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