R help - Nov 2009 - different values under the x axis labels

Hi everyone,
I am a complete beginner, no mathematician and I would like to ask You for help
since this is my first experience with such a software like "R".

I have a .xls source file from 1 to 3600 seconds, each second has a noise level
value. (attached in case you are interested)
Imported as "Dataset".
I would like to make a graph: Here is what I have been able to use:

par(mar = c(5, 4, 4, 2) + 0.1)
plot(Dataset, type="l", xlab="?as [s]", ylab=
expression(paste("Hladina akustick?ho tlaku","     ",
bold(L[pA])," [dB]")), col="blue", xlim=c(0,3600),
ylim=c(35,60), xaxs = "i", yaxs = "i",cex.main=1.5,
cex.lab=1.2,xaxt="n")
abline(h=43,col="red",lwd=2)
title(main = "M?sto p??jmu 3, ?as 9:00 -
10:00",font.main="2", line=3)
axis(3, xaxp=c(0, 3600, 12), mgp=c(3,0.6,0))
axis(1, xaxp=c(0, 3600, 12), mgp=c(3,0.6,0))
smartlegend(x="left",
y="top",c(expression(paste(bold(L[eqA.1h]),bold(" [dB]"))),
expression(paste(bold(L[pA]),bold("
[dB]")))),col=c("red","blue"), lty= 1:1,lwd=2,
y.intersp= 1.1, bg="grey96",cex=1.1)

I succeeded in changing scale on both "X" after 300 seconds and now I
would like to swap the value 300, 600, 900 (seconds)of the top axis to 5, 10, 15
(minutes) and so on OR
the best of all would be to have these numbers (minutes) under the original
bottom x axis values in different color.


Thank you very much for any ideas.

Lubo

I have a .xls source file from 1 to 3600 seconds, each second has a noise 
level value. (attached in case you are interested)
Imported as "Dataset".
I would like to make a graph: Here is what I have been able to use:

par(mar = c(5, 4, 4, 2) + 0.1)
plot(Dataset, type="l", xlab="?as [s]", ylab=
expression(paste("Hladina
akustick?ho tlaku","     ", bold(L[pA])," [dB]")),
col="blue",
xlim=c(0,3600), ylim=c(35,60), xaxs = "i", yaxs =
"i",cex.main=1.5,
cex.lab=1.2,xaxt="n")
abline(h=43,col="red",lwd=2)
title(main = "M?sto p??jmu 3, ?as 9:00 -
10:00",font.main="2", line=3)
axis(3, xaxp=c(0, 3600, 12), mgp=c(3,0.6,0))
axis(1, xaxp=c(0, 3600, 12), mgp=c(3,0.6,0))
smartlegend(x="left",
y="top",c(expression(paste(bold(L[eqA.1h]),bold("
[dB]"))), expression(paste(bold(L[pA]),bold("
[dB]")))),col=c("red","blue"),
lty= 1:1,lwd=2, y.intersp= 1.1, bg="grey96",cex=1.1)

I succeeded in changing scale on both "X" after 300 seconds and now I
would
like to swap the value 300, 600, 900 (seconds)of the top axis to 5, 10, 15 
(minutes) and so on OR
the best of all would be to have these numbers (minutes) under the original 
bottom x axis values in different color.

Does this get you started?

Dataset=data.frame(time=1:3600,noise=15*abs(rnorm(3600)))
plot(Dataset, type="l", xlab="?as [s]", ylab=
expression(paste("Hladina
akustick?ho tlaku","     ", bold(L[pA])," [dB]")),
col="blue",
xlim=c(0,3600), ylim=c(35,60), xaxs = "i", yaxs =
"i",cex.main=1.5,
cex.lab=1.2,xaxt="n")
abline(h=43,col="red",lwd=2)
title(main = "M?sto p??jmu 3, ?as 9:00 -
10:00",font.main="2", line=3)
# axis(3, xaxp=c(0, 3600, 12), mgp=c(3,0.6,0))
axis(3, at=seq(0,3600,300), labels=seq(0,60,5))
axis(1, xaxp=c(0, 3600, 12), mgp=c(3,0.6,0))


Rob