[R] plot with different color

[array chip]

Hi how can I plot a series of number as a line, but
with lines above a threshould as one color, and with
lines below the threshold as another color. for
example, a numeric vector: rnorm(1:100), and plot
these numbers in the original order, but lines above
the horizontal line at 0 are in red, and lines below
in blue?

Thanks