Overlapping x - axis lables

Dear R helpers

Suppose I am plotting a simple scatter plot where no of paired observations (x,y) are say 100.


month          length
1                   10
2                   12
3                   17
4                   21
5                   13
..........................
..........................
..........................

100              16



when i run the command

plot(month, length)

the required plot is generated. But the problem is that on  the X - axis, month values are also printed and it becomes difficult to read the month values on the graph.
  
(Actually I am working on a stacked graph where I am facing this problem.)

So, how to display only few month values (say 1, 5, 10 and so on) without affecting the original graph i.e. the graph should display all the data 100 points but on x - axix only few month values should be displayed in order to improve the readability.