thanks for your answer.
I am afraid I do not understand the second part. could you guide me further what to do in my case ?
This is my plotting code (reduced, to make it more transparrent):
plot(ACET$Temp_UP,ACET$Salt_UP..M. ,ylim=c(0,5.2),col=c("black"), pch= 15,xlim= c(0,125))
points(SRB_S$Temp_UP,SRB_S$Salt_UP..M. ,ylim=c(0,5.2), pch= 1,col=c("black"), xlim= c(0,125))
points(METH$Temp_UP,METH$Salt_UP..M. ,ylim=c(0,5.2), pch= 17,col=c("black"), xlim= c(0,125))
Many thanks in advance!