2 barplots in the same graph

Thanks All!

I combine your answers and post the code here again, if later somebody need
it.

Actually, what I want is:
  

x1
    

[1]  1  2 10
  

x2
    

[1] -3  5 -8
  

barplot(x1,col="white",border="red")
barplot(x2,col="white",border="green",add=T)
    

So that, the two plots even share the same x-axis.

But, It comes another question: >
barplot(x2,col="white",border="green",add=T), because there are 2 numbers
are negative, some part of the bar is missing. Is there any automatic
function to make the axes fit very well for both bars?

  

And, suppose I want to add another command: axis(1, 0:20) to draw x-axis. I
notice it  increase like: 0,1,2,3,...20; how can I make it 0,5,15,20?

  

Thanks!