UserPc and PostingsPc cointain "," and "%" so read.csv interprets them as strings (which it reads as factors) rather than numbers. You'll see this if you run str(myData). If you want to plot them, you need to convert them into numbers, which looking at your data requires replacing "," with "." and removing the "%". gsub is a useful function for this, and it's convenient to make the whole operation its own function. Something like this:
MyData <- read.csv(file="data.csv", header=TRUE, sep=";",stringsAsFactors = FALSE)
#write a function that removes all "%" from a string converts "," to "." and returns a numeric
#divide by 100 because it's a percentage
convert <- function(stringpct){
as.numeric(gsub("%","",gsub(",",".",stringpct)))/100
}
MyData$UserPc <- convert(MyData$UserPc)
MyData$PostingsPc <- convert(MyData$PostingsPc)
attach(MyData)
plot(UserPc,PostingsPc,ylab = "Users", xlab= "Postings",type="l")