[R] how to plot 2d matrix as coloured squares?

[Jonas Stein]

i have a matrix like 

    x1  x2  x3
y1  2   34  5656
y2  34  434 342
y3  234 43  34

i want to plot these values like here
http://www.almob.org/content/2/1/12/figure/F5?highres=y

The rainbow function could calculate a colour for each value.
But how van i generate the square pattern?
 
Kind regards,

-- 
Jonas Stein <news at jonasstein.de>