pan, in y-axis units (0-1).

        Returns
        -------
        `~matplotlib.patches.Polygon`
            Vertical span (rectangle) from (xmin, ymin) to (xmax, ymax).

        Other Parameters
        ----------------
        **kwargs : `~matplotlib.patches.Polygon` properties

        %(Polygon:kwdoc)s

        See Also
        --------
        axhspan : Add a horizontal span across the Axes.

        Examples
        --------
        Draw a vertical, green, translucent rectangle from x = 1.25 to
        x = 1.55 that spans the yrange of the Axes.

        >>> axvspan(1.25, 1.55, facecolor='g', alpha=0.5)

        r 