Add Lines to a Panel
USAGE: 
panel.abline(coef, ...)
panel.abline(a, b, ...)
panel.abline(reg, ...)
panel.abline(h=, v=, ...)
coef:
vector containing the intercept a and slope b of the line y=a+b*x.
a,b:
intercept and slope as above.
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.
h:
vector of y-coordinates for horizontal lines across plot.
v:
vector of x-coordinates for vertical lines across plot.
...:
graphical parameters may be specified to control the line(s). The trellis parameter add.line provides default values of the line width (lwd), line type (lty), and color (col) parameters.
SIDE EFFECTS:
The effect of panel.abline is that the line y=a+b*x or the specified horizontal and vertical lines are drawn across the current plot.
SEE ALSO:
The function panel.abline is similar to abline but it does not issue a warning message if the line does not intersect the plot and it uses the Trellis rendering parameters.
EXAMPLES: 
     # line with 0 intercept and slope 1
panel.abline(0,1)
     # line produced by least-squares fit
panel.abline(lsfit(longley.x[,1],longley.y))
     # dotted vertical lines at x==0 and 10
panel.abline(v=c(0,10),lty=2)