Add a line from another data.frame to qplot

Question

Is it possible to add a line (such as an additional time series) to an already existing plot? I know how to add horizontal or vertical lines, but how can I add from other data.frames?

 q_myplot<-qplot(datefield,myvalue,data=mydf,geom=c("line"),colour=category) +opts(axis.title.x = theme_blank()) + scale_x_date(major="2 years")

is my basic plot, showing three different time series grouped by category. Is there a way to add another line to the plot for example by using layers? Of course I could add this additional data to mydf using another category, but I wonder if there's a better idea out there.

Answer 1

by using ggplot() instead of qplot(), you can have more flexibility. here is a minimal example using two datasets:

d1 <- data.frame(x1=rep(1:10,3), y1=rnorm(10*3), g1=gl(3,10,labels=letters[1:3]))
d2 <- data.frame(x2=rep(1:10,3), y2=rnorm(10*3), g2=gl(3,10, labels=letters[4:6]))

ggplot() + 
  geom_line(aes(x1, y1, colour=g1), d1) +  
  geom_line(aes(x2, y2, colour=g2), d2)