docs: def line(self, x, label=None, y='bottom', color='grey', ax=None, **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.
          '''
          if ax is None:
              ax = plt
              y0, y1 = ax.ylim()
          else:
              y0, y1 = ax.get_ylim()
          ax.axvline(x, color=color, **kwargs)
          if label is not None:
              verticalalignment = 'bottom'
              if y == 'bottom':
                  y = y0 + (y1 - y0) / 25.
              if y == 'top':
                  verticalalignment = 'top'
                  y = y0 + (y1 - y0) * 24 / 25.
              ax.annotate('\n' + label, (x, y), rotation=90,
                          verticalalignment=verticalalignment)