rotate X axis labels 45 degrees on grouped bar plot R

Question

How can I rotate the X axis labels 45 degrees on a grouped bar plot in R?

I have tried the solution suggested here but got something very messy, the labels seem to have been added multiple times (only showing the axis part to protect data privacy):

This solution (gridBase) was also unsuccessful for me, for some reason I get the following error:

"Cannot pop the top-level viewport (grid and graphics output mixed?)"

PS. Most people seem to recommend this solution in R base but I am stuck with that too because I don't understand what data they are referring to (I need some kind of example data set to understand new command lines...).

Are these solutions not working because my barplot is a grouped barplot? Or should it work nevertheless? Any suggestions are welcome, I have been stuck for quite some time. Thank you.

[edit] On request I am adding the code that I used to generate the picture above (based on one of the text() solutions):

data <- #this is a matrix with 4 columns and 20 rows;
        #colnames and rownames are specified.
        #the barplot data is grouped by rows

lablist <- as.vector(colnames(data))

barplot(data, beside=TRUE, col=c("darkred","red","grey20","grey40"))
text(1:100, par("usr")[1], labels=lablist, srt=45, pos=1, xpd=TRUE)

Answer 1

I am not a base plot proficient, so maybe my solution is not very simple. I think that using ggplot2 is better here.

def.par <- par(no.readonly = TRUE)

## divide device into two rows and 1 column 
## allocate figure 1  for barplot
## allocate figure 2 for barplot labels
## respect relations between widths and heights

nf <- layout(matrix(c(1,1,2,2),2,2,byrow = TRUE), c(1,3), c(3,1), TRUE)
layout.show(nf)

## barplot 
par(mar = c(0,1,1,1))
set.seed(1)
nKol <- 8  ## you can change here but more than 11 cols 
           ## the solution is not really readable
data <- matrix(sample(1:4,nKol*4,rep=TRUE),ncol=nKol)
xx <- barplot(data, beside=TRUE,
              col=c("darkred","red","grey20","grey40"))

## labels , create d ummy plot for sacles
par(mar = c(1,1,0,1))
plot(seq_len(length(xx)),rep(1,length(xx)),type='n',axes=FALSE)
## Create some text labels 
labels <- paste("Label", seq_len(ncol(xx)), sep = " ")
## Plot text labels with some rotation at the top of the current figure
text(seq_len(length(xx)),rep(1.4,length(xx)), srt = 90, adj = 1,
     labels = labels, xpd = TRUE,cex=0.8,srt=60,
     col=c("darkred","red","grey20","grey40"))

par(def.par)  #- reset to default