Add an electrode to the plot.

        Args:
            electrode: An electrode. All electrodes satisfying the
                AbstractElectrode interface should work.
            label: A label for the electrode. If None, defaults to a counting
                system, i.e. 'Electrode 1', 'Electrode 2', ...

def add_electrode(self, electrode, label=None):
        """
        Add an electrode to the plot.

        Args:
            electrode: An electrode. All electrodes satisfying the
                AbstractElectrode interface should work.
            label: A label for the electrode. If None, defaults to a counting
                system, i.e. 'Electrode 1', 'Electrode 2', ...
        """
        if not label:
            label = "Electrode {}".format(len(self._electrodes) + 1)
        self._electrodes[label] = electrode