plot question

I have the following simple situation:

tt <- data.frame(c(0.5, 1, 0.5))
names(tt) <- "a"
plot(tt$a, type = 'o')

gives the following plot ('I' and '.' represent the axis):

I
I
I     X
I
I
I X       X
I...........
  1   2   3

what do I have to change to get the following:


I
I
I          X
I
I
I      X       X
I.....................
       1   2   3

i.e. the plot-region should be widened at the left and right side