[R] surpressing tickmarks / labels x-as for two sets of boxplot (plotted as stacked boxplots)

[Karin]

Hello,

I am trying to plot two sets of boxplots together. These are estimates of two 
experiments and seven factors.
The results of the two experiments I want to plot as boxplots stacked to each 
other.
Therefore I plot first the results of the first experiment; and next with the 
add option the second set of boxplots.
The boxplots are plotted at 'at = 1:7 - 0.15 for the first experiment and at=1:7 
+ 0.15 for the second.
I surpress plotting the tickmarks and labels succesfully for the first boxplot 
with xaxt="n".
But for the second this does not work! 
I want to plot the tickmarks and labels  at position at=1:7, as below using the 
axis function.
But with this code also tickmarks and labels are plotted at position 
at=1:7+0.15.

boxplot(coefs ~ factor, data = temp,
        boxwex = 0.25, at = 1:7 - 0.15,
        subset = experiment == "first", col = "red",
         xlab = "factor",xaxt="n",
        ylab = "individual estimates")

boxplot(coefs ~ factor, data = temp, naxt="n",add = TRUE,
        boxwex = 0.25, at = 1:7 + 0.15, 
        subset = experiment == "second", col = "green")

axis(at=1:7,side=1,c("fac1","fac2","fac3","fac4","fac5","fac6","fac7"))

legend(6,-0.5, c("experiment1", "experiment2"),
       fill = c("red", "green"))

Does anyone know how I can surpress these labels for the second boxplot?

Thanks in advance,

Karin