Overlay boxplot and scatter.smooth line

Hi,

I'd like to plot a boxplot and use a scatter.smooth line plot as an overlay
for this plot.
This works except for the problem that the x-axis ticks for both plots are
differently spaced:
The boxplot ticks are closer and the space between the outer most boxplots
and the plot region (box) is larger for boxplots than for the
scatter.smooth plot. Is there any plot-option that can be changed to
produce the desired plot.

Here an example to illustrate what problem I am facing:

x <- c(rep(1,100),rep(2,100),rep(3,100))
y <- c(rnorm(100,1),rnorm(100,2),rnorm(100,3))

boxplot(y~x)
par(new=TRUE)
scatter.smooth(x, y, xaxt="n", yaxt="n", ann=FALSE)

Moreover, I'd like to plot just the smoothed line only and not the
datapoints.