Q:

Plot multiple lines (data series) each with unique color in R

I am fairly new to R and I have the following queries :
I am trying to generate a plot in R which has multiple lines (data series). Each of these lines is a category and I want it to have a unique color. 
Currently my code is setup in this way :
First, I am creating an empty plot :
plot(1,type='n',xlim=c(1,10),ylim=c(0,max_y),xlab='ID', ylab='Frequency')

Then for each of my category, I am plotting lines in this empty plot using a "for" loop like so :
for (category in categories){
lines(data.frame.for.this.category, type='o', col=sample(rainbow(10)), lwd=2)
}

There are 8 categories here, and so there are 8 lines produced in the plot. As you can see, I am trying to sample a color from the rainbows() function to generate a color for each line. 
However, when the plot is generated, I find that there are multiple lines which have the same color. For instance, 3 of those 8 lines have green color. 
How do I make each of these 8 lines have a unique color ? 
Also, how do I reflect this uniqueness in the legend of the plot ? I was trying to lookup the legend() function, however it was not clear which parameter I should use to reflect this unique color for each category ? 
Any help or suggestions would be much appreciated.

A:

If your data is in wide format matplot is made for this and often forgotten about:
 dat <- matrix(runif(40,1,20),ncol=4) # make data
 matplot(dat, type = c("b"),pch=1,col = 1:4) #plot
 legend("topleft", legend = 1:4, col=1:4, pch=1) # optional legend

There is also the added bonus for those unfamiliar with things like ggplot that most of the plotting paramters such as pch etc. are the same using matplot() as plot().

A:

If you would like a ggplot2 solution, you can do this if you can shape your data to this format (see example below)
# dummy data
set.seed(45)
df <- data.frame(x=rep(1:5, 9), val=sample(1:100, 45), 
                   variable=rep(paste0("category", 1:9), each=5))
# plot
ggplot(data = df, aes(x=x, y=val)) + geom_line(aes(colour=variable))

A:

You have the right general strategy for doing this using base graphics, but as was pointed out you're essentially telling R to pick a random color from a set of 10 for each line. Given that, it's not surprising that you will occasionally get two lines with the same color. Here's an example using base graphics:
plot(0,0,xlim = c(-10,10),ylim = c(-10,10),type = "n")

cl <- rainbow(5)

for (i in 1:5){
    lines(-10:10,runif(21,-10,10),col = cl[i],type = 'b')
}

Note the use of type = "n" to suppress all plotting in the original call to set up the window, and the indexing of cl inside the for loop.

