Plot all time series in the group.

def plot(self, overlay=True, **labels): # pragma: no cover
        '''Plot all time series in the group.'''
        pylab = LazyImport.pylab()
        colours = list('rgbymc')
        colours_len = len(colours)
        colours_pos = 0
        plots = len(self.groups)
        for name, series in self.groups.iteritems():
            colour = colours[colours_pos % colours_len]
            colours_pos += 1
            if not overlay:
                pylab.subplot(plots, 1, colours_pos)
            kwargs = {}
            if name in labels:
                name = labels[name]
            if name is not None:
                kwargs['label'] = name
            pylab.plot(series.dates, series.values, '%s-' % colour, **kwargs)
            if name is not None:
                pylab.legend()
        pylab.show()