

   DDrraaww aa hhoorriizzoonnttaall lliinnee

        yline(y, ...)

   AArrgguummeennttss::

          y: Value on y axis specifying location of horizontal
             line.

        ...: Any ploting options for abline function.

   EExxaammpplleess::

        plot(ozone$x[,2],ozone$y) #plot ozone longitude vs ozone value
        yline(40) #plots horizontal line at ozone value of 40

