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        abline(a, b, ...)
        abline(h=, ...)
        abline(v=, ...)
        abline(coef=, ...)
        abline(reg=, ...)

   DDeessccrriippttiioonn::

        This function adds one or more straight lines through
        the current plot.  The first form above specifies the
        line in intercept/slope form (alternatively `a' can be
        specified on its own and is taken to contain the slope
        and intercept in vector form).  The `h=' and `v=' forms
        draw horizontal and vertical lines at the specified
        coordinates.  The `coef' form specifies the line by a
        vector containing the slope and intercept.  `reg' is a
        regression object which contains `reg$coef'.  If it is
        of length 1 then the value is taken to be the slope of
        a line through the origin, otherwise, the first 2 val-
        ues are taken to be the slope and intercept.

        The graphical parameters `col' and `lty' can be speci-
        fied as arguments to `abline'; see `par' for details.

   SSeeee AAllssoo::

        `lines' and `segments' for connected and arbitrary
        lines given by their endpoints.  `par'.

   EExxaammpplleess::

        data(cars)
        z <- lm(dist ~ speed, data = cars)
        plot(cars)
        abline(z)

