[R] reducing the number of x-axis lables in a bwplot while plotting all boxes

[Jeff D. Hamann]

I apologize if this is somewhere in the archives, but I can't seem to find
a solution to this question.

I've been trying to plot a bwplot:

print(
      bwplot( n.pareto ~ as.factor(gen) |  mut.rate * n.pop,
             data=p6,
             horizontal=FALSE,
             box.ratio=0.75,
             cex=0.6,
             xlim=c(-1,51),
             ylim=c(-1,500),
             layout=c(3,3),
             index.cond=list(c(1,2,3),c(3,2,1)),
             panel = function(...) {
               panel.grid(v = -1, h = -1)
               cex=0.05
               panel.bwplot(...)
               panel.abline(h=486,lty=2)
             },
             main="Non-dominated Pareto Optimal Solutions",
             ylab="Non-dominated Solutions in the Pareto Archive",
             xlab="Generation" )
      )

which works "fine", but... I have tried almost every combination of
pertinent arguments for reducing the number of ticks/lables while still
plotting all the boxes in each panel.  The levels of "gen" are:

> levels( as.factor(p6$gen ))
 [1] "20"   "40"   "60"   "80"   "100"  "120"  "140"  "160"
 [9] "180"  "200"  "220"  "240"  "260"  "280"  "300"  "320"
[17] "340"  "360"  "380"  "400"  "420"  "440"  "460"  "480"
[25] "500"  "520"  "540"  "560"  "580"  "600"  "620"  "640"
[33] "660"  "680"  "700"  "720"  "740"  "760"  "780"  "800"
[41] "820"  "840"  "860"  "880"  "900"  "920"  "940"  "960"
[49] "980"  "1000"
>

which creates a "smear" of labels across the x axis. My problem is that I
would like to only print five or 10 of the labels (i.e.
20,100,200,...,900) or something to make the labels readable and still
plot all the boxes in each panel.

Is there something I'm missing as a result  conditioning on two factors. I
feel like such a newbie about this... ugh... Any hints, please?

Thanks,
Jeff.

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