Is it possible to group by two columns? So the cross product is drawn by geom_point() and geom_smooth()?
frame <- data.frame(series <- rep(c('a', 'b'), 6), sample <- rep(c('glass',
'water', 'metal'), 4), data <- c(1:12))
ggplot(frame, aes()) # ...
Such that the points 6 and 12 share a group, but not with 3.