Returns or sets (if a value is provided) the chart's x-axis label.

        :param str x_label: If given, the chart's x_label will be set to this.
        :rtype: ``str``

def x_label(self, x_label=None):
        """Returns or sets (if a value is provided) the chart's x-axis label.

        :param str x_label: If given, the chart's x_label will be set to this.
        :rtype: ``str``"""

        if x_label is None:
            return self._x_label
        else:
            if not isinstance(x_label, str):
                raise TypeError("x_label must be str, not '%s'" % str(x_label))
            self._x_label = x_label