en 0 and 1, 0 being the far left of the plot, 1 the
            far right of the plot.

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

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

            %(Line2D:kwdoc)s

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

        Examples
        --------
        * draw a thick red hline at 'y' = 0 that spans the xrange::

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

        * draw a default hline at 'y' = 1 that spans the xrange::

            >>> axhline(y=1)

        * draw a default hline at 'y' = .5 that spans the middle half of
          the xrange::

            >>> axhline(y=.5, xmin=0.25, xmax=0.75)
        