[R] how to construct stripchart with coincident points centered

[Berta]

Hi R-users,
I am using stripchart with coincident points,

y <-rbinom(100, 3, 0.5)
stripchart(y, method="stack", pch="o", vertical=TRUE)

But the result is not centered in the sense that if a value (say value 0) is 
repeated 7 times, the first point is ploted in the middle and the rest at 
its right side, in stead of ploting 3 at its right and 3 at its left.  Can 
anybody help?

Thanks,
Berta.