[R] Boxplot by factors

[msck9@mizzou.edu]

Dear all, 
 I have the following data format


 cellnumber    force
 	1	100
	1	230
	1	100
	1 	200
	1	130
	1	210
	2	179
	2	298
	2	400
	2	500
	2	600
	...........
I want to make a boxplot of the force according to the cellnumber. Here
the cellnumber is actually a factor. It has 1, 2 two levels. How can I
do that using boxplot? 

Thanks in advance
Ming