Add a time scale to the plot.

        :param timescale: the timescale to be added
        :type  timescale: :class:`~aeneas.plotter.PlotTimeScale`
        :raises: TypeError: if ``timescale`` is not an instance of :class:`~aeneas.plotter.PlotTimeScale`

def add_timescale(self, timescale):
        """
        Add a time scale to the plot.

        :param timescale: the timescale to be added
        :type  timescale: :class:`~aeneas.plotter.PlotTimeScale`
        :raises: TypeError: if ``timescale`` is not an instance of :class:`~aeneas.plotter.PlotTimeScale`
        """
        if not isinstance(timescale, PlotTimeScale):
            self.log_exc(u"timescale must be an instance of PlotTimeScale", None, True, TypeError)
        self.timescale = timescale
        self.log(u"Added timescale")