# [R] polygon

Paul Murrell p.murrell at auckland.ac.nz
Tue Apr 20 23:32:29 CEST 2004

```Hi

mike.campana at freesurf.ch wrote:
> Dear all
>
> In order to clearly mark values wich are larger than a treshold value, I
> would like to color the surface below the line given by plot (yy~xx). To
> color is only the surface between abline (treshold) and yy if they are
> larger than the specific limit. I guess I can use the function polygon,
> but I can not find any valuable solution.
> I'm grateful to you for an advice or an example.
> Mike
>
> xx <- c(1:100)
> yy <- rnorm(100)
> plot (yy~xx,type="l")
> abline (h=0.5,col="red")
> #?? how can I use polygon()
> #in order to color surface below yy value and > abline??
> polygon(xx,yy,col="gray")

You can do it by "brute force" as follows:

xx <- c(1:100)
yy <- rnorm(100)
# Set up plot but draw nothing
plot (yy ~ xx, type="n", axes=FALSE)
# Some constants
n <- 100
hline <- 0.5
# Fill in grey beneath yy
polygon(c(xx[1], xx, xx[n]), c(min(yy), yy, min(yy)), col="grey")
# Fill in white beneath abline
usr <- par("usr")
rect(usr[1], usr[3], usr[2], hline, col="white", border=NA)
# Redraw all of yy line
lines(xx, yy)
# Draw red abline
abline (h=0.5,col="red")
# Draw bounding box and axes
box()
axis(1)
axis(2)

Or you could calculate all intercepts of yy line with the abline, but
that's probably not worth the effort.

Paul
--
Dr Paul Murrell
Department of Statistics
The University of Auckland
Private Bag 92019
Auckland
New Zealand
64 9 3737599 x85392
paul at stat.auckland.ac.nz
http://www.stat.auckland.ac.nz/~paul/

```