R help - Mar 2010 - simple line graphics, labels and legend

Dear users,

I think my questions are pretty simple, but I got lost in the hundreds 
of par() and plot() arguments and plot functions, so I don't know in 
which direction I should go.

Here is my sample dataset:
test <- structure(list(DIET = structure(c(1L, 1L, 1L, 1L, 1L, 3L, 3L, 
3L, 3L, 3L, 2L, 2L, 2L, 2L, 2L, 4L, 4L, 4L, 4L, 4L), .Label = c("G", 
"GG", "L", "LG"), class = "factor"),
SILIKATG = c(0.0152, 0.0161,
0.0141, 0.0131, 0.0108, 0, 0, 0, 0, 1e-04, 0.0056, 0.0078, 0.0072, 
0.0047, 0.0091, 3e-04, 5e-04, 0, 0.0013, 0.0012)), .Names = c("DIET", 
"SILIKATG"), row.names = c(1L, 2L, 3L, 4L, 5L, 31L, 32L, 33L, 34L,
35L,
61L, 62L, 63L, 64L, 65L, 101L, 102L, 103L, 104L, 105L), class = 
"data.frame")

1. The first plot I want to do is:
plot(test$SILIKATG, ylim=c(0,0.02), type="l")

It looks the way I want to, but my question is:*
How to change the labels on the x-axis? I would like to have the groups 
("G", "GG", "L", "LG") labeled on the
x-axis instead of the "5", "10",
"15", "20" of "Index". And if possible with ticks
to delimit the groups. *

I've tried:    plot(test$SILIKATG~test$DIET, ylim=c(0,0.02), 
type="l")        But I get boxplots...

2. The second graph is an alternative, and I would like to use both for now:
I first modify the data:
selectG <- test[test$DIET=="G",]; selectG$DIET <-
factor(selectG$DIET,
levels="G")
I do it for each group and:
plot(selectG$SILIKATG, type="p", ylim=c(0,0.02), pch=1,
col="black");
points(selectGG$SILIKATG, type="p", ylim=c(0,0.02), pch=2,
col="red")
And so on for each group, and then add the legend with:
legend(x="topright", legend=c("G", "GG",
"L", "LG"), pch=1:4,
col=c("black", "red", "green", "blue"))

My question here is: *
Isn't there an easier way to do all of it?!
I'm quite sure there's a way where I don't have to select each group
and
plot it separately. And I guess something more automatic also exists 
(e.g. the legend so that I don't have to give the names of the groups 
and the colors and symbols used etc).

*Thank you all in advance for your help.
Regards,
Ivan

-- 
Ivan CALANDRA
PhD Student
University of Hamburg
Biozentrum Grindel und Zoologisches Museum
Abt. Säugetiere
Martin-Luther-King-Platz 3
D-20146 Hamburg, GERMANY
+49(0)40 42838 6231
ivan.calandra@uni-hamburg.de

**********
http://www.for771.uni-bonn.de
http://webapp5.rrz.uni-hamburg.de/mammals/eng/mitarbeiter.php


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Hi Ivan,
You can get your first plot this way:

plot(test$SILIKATG, ylim=c(0,0.02),
type="l",xaxt="n",xlab="Group")
axis(1,at=c(5,10,15,20),labels=sort(unique(test$DIET)))

I was going to suggest using reshape to get the following matrix, but I 
just couldn't work out how to do it.

test.wide<-cbind(test$SILIKATG[test$DIET=="G"],
  test$SILIKATG[test$DIET=="GG"],
  test$SILIKATG[test$DIET=="L"],
  test$SILIKATG[test$DIET=="LG"])
matplot(test.wide,pch="")
text(rep(1:5,4),test.wide,rep(c("G","GG","L","LG"),each=5),
  col=rep(2:5,each=5))

Jim