loat, default: 1
            Should be between 0 and 1, 0 being the bottom of the plot, 1 the
            top of the plot.

        Returns
        -------
        `~matplotlib.lines.Line2D`

        Other Parameters
        ----------------
        **kwargs
            Valid keyword arguments are `.Line2D` properties, except for
            'transform':

            %(Line2D:kwdoc)s

        See Also
        --------
        vlines : Add vertical lines in data coordinates.
        axvspan : Add a vertical span (rectangle) across the axis.
        axline : Add a line with an arbitrary slope.

        Examples
        --------
        * draw a thick red vline at *x* = 0 that spans the yrange::

            >>> axvline(linewidth=4, color='r')

        * draw a default vline at *x* = 1 that spans the yrange::

            >>> axvline(x=1)

        * draw a default vline at *x* = .5 that spans the middle half of
          the yrange::

            >>> axvline(x=.5, ymin=0.25, ymax=0.75)
        r