Hi Benoit, I'm not a specialist of ggplot2, but I will try to help. You may obtain more --interesting-- answers on the ggplot2 mailing list. This said, let's go.
To solve your problem, I would suggest to 1. change the form of the data frame (using the reshape library) in order to have one variable for Temp, one for the different Xs, and one for their y values. 2. add a new variable for the different molecules. 3. then plot # change the format of the data frame library(reshape) mdat <- melt(THT_N2_ATGMS, id="Temp") # add the molecule variable mdat$mol <- 'other' mdat$mol[mdat$variable %in% c('X22','X44')] <- 'CO2' mdat$mol[mdat$variable %in% c('X43','X45')] <- 'AA' #plot library(ggplot2) p <- ggplot(data=mdat, aes(x=Temp, y=value, colour=mol, linetype=variable)) + geom_line() p that's it. HTH Matthieu ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.