docs: def line(self, x, label=None, y='bottom', color='grey', **kwargs):
          '''
          Creates a vertical line in the plot.
  
          :param x:
              The x coordinate of the line. Should be in the same units
              as the x-axis.
          :param string label:
              The label to be displayed.
          :param y:
              May be 'top', 'bottom' or int.
              The y coordinate of the text-label.
          :param color color:
              The color of the line.
          '''
          super(DiffPlotter, self).line(x, label, y, color, self.ax1, **kwargs)
          super(DiffPlotter, self).line(x, '', 0, color, self.ax2, **kwargs)