Plot Line in Intercept-Slope Form
DESCRIPTION:
Adds a line to a plot, often a regression line.
USAGE: 
abline(a, b)
abline(coef)
abline(reg)
abline(h=, v=)
REQUIRED ARGUMENTS:
one of the forms below must be specified.
a,b:
intercept and slope as above.
coef:
vector containing the intercept a and slope b of the line y=a+b*x.
reg:
a regression object, such as returned by lsfit. Specifically, reg$coef, if it is of length 2, will be used to define the intercept and slope of the line. If reg$coef is of length 1, it is treated as the slope of a fit through the origin. The first two elements are used if reg$coef has length greater than 2.
h=:
vector of y-coordinates for horizontal lines across plot. Missing values are not accepted. You may give only one of h and v.
v=:
vector of x-coordinates for vertical lines across plot. Missing values are not accepted.
OPTIONAL ARGUMENTS:

Graphical parameters may also be supplied as arguments to this function (see par).

SIDE EFFECTS:
The effect of abline is that the line y=a+b*x or the specified horizontal and vertical lines are drawn across the current plot. A warning is given if a line does not intersect the plot.
DETAILS:
abline is a generic function; currently there is only the default method available.
SEE ALSO:
lsfit , par , segments .
EXAMPLES: 
# line with 0 intercept and slope 1
abline(0, 1)


# line produced by least-squares fit
abline(lsfit(longley.x[, 1], longley.y))


# dotted vertical lines at x==0 and 10
abline(v = c(0, 10), lty = 2)