[R] plotting a grid with color on a map

[dxc13]

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 
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