Hi, to be concise, let me start with my problem: I have a scatterplot that I want to fit an envelope curve to. The picture of the scatterplot is below. <http://r.789695.n4.nabble.com/file/n4673965/day_DryPond1_1_rid03.jpg> I have 140 of these plots that I need to compare. Rather than to visually compare all the plots, I would like to compare the parameters of the curves enveloping them. Possible parameters could be the x-value when the curves starts dropping down, the y-value that is the asymptote of the enveloping curve and some parameter describing the curve itself. After some googling, I still have no idea how to approach this. Could anybody please give me a hint where to start? I attached the data. The plot was generated using the code below(I plotted the 2nd column against columns 15, 16, 17. ) plot(output[,2],(1-output[,15]),xlab="Precip [inch]", ylab="Eff.",ylim=c(-1,1),xlim=c(0,8),pch=18) points(output[,2],(1-output[,16]),col=2,pch=18) points(output[,2],(1-output[,17]),col=3,pch=18) DryPond1_1_rid03_24.txt <http://r.789695.n4.nabble.com/file/n4673965/DryPond1_1_rid03_24.txt> Any help would be much appreciated. Thank you! Frauke -- View this message in context: http://r.789695.n4.nabble.com/Envelope-curve-for-scatterplot-tp4673965.html Sent from the R help mailing list archive at Nabble.com.