plotting a grid with color on a map

Hi all,
I have posted similar questions regarding this topic, but I just can't seem
to get over the hump and find a straightforward way to do this.  I have
attached my file as a reference.
Basically, the attached file is a 5 degree by 5 degree grid of the the world
(2592 cells), most of them are NA's.  I just want to be able to plot this
grid over a world map and color code the cells.  For example, if a cell has
a temperature less than 20 degrees it will be blue, 21 to 50 green color,
51-70 orange, 71+ red colored cells.  For any NAs, they should be colored
white.

I know how to create a map of the world using map() and add a grid to it
using map.grid(), but I can't color code the cells the way I need.  Is there
a way to do this in R?

Thanks again.
dxc13 http://www.nabble.com/file/p23514804/time1test.txt time1test.txt 

Hi all,
I have posted similar questions regarding this topic, but I just can't seem
to get over the hump and find a straightforward way to do this.  I have
attached my file as a reference.
Basically, the attached file is a 5 degree by 5 degree grid of the the world
(2592 cells), most of them are NA's.  I just want to be able to plot this
grid over a world map and color code the cells.  For example, if a cell has
a temperature less than 20 degrees it will be blue, 21 to 50 green color,
51-70 orange, 71+ red colored cells.  For any NAs, they should be colored
white.

I know how to create a map of the world using map() and add a grid to it
using map.grid(), but I can't color code the cells the way I need.  Is there
a way to do this in R?
  

It was the NAs that fooled color2D.matplot. This gets your colors, 
although not exactly what you want. Look at the help for color2D.matplot 
to get that. I think fiddling with the x and y limits on the map() call 
will get the positions right.

temp1<-read.table("time1test.dat",header=TRUE)
library(plotrix)
# reverse the row order, as color2D.matplot reverses it
color2D.matplot(temp1[36:1,],c(0.5,1),c(0.5,0),c(1,0),axes=FALSE)
# don't erase the above plot
par(new=TRUE)
# do a "ghost" plot with just the axes
plot(0,xlim=maplim[1:2],ylim=maplim[3:4],type="n")
# add the map on top in black
map(add=TRUE,col="black")

Jim