plot points in front of lines for each group/ggplot2 equivalent of type=“o”

Question

Suppose I want to plot a graph with both points and lines where points appear in front of their corresponding lines in each group. In particular, I want group 1 to be plotted with red filled points, where the points are connected by a line, but group 2 to be plotted with (just) a blue line, but I want group 2 to be plotted over group 1. For example, in base graphics:

set.seed(101)
dd <- data.frame(x=rep(1:10,2),
         y=rep(1:10,2),
                 f=factor(rep(1:2,each=10)))
dd$y[11:20] <-     dd$y[11:20] + rnorm(10)
d1 <- subset(dd,f=="1")
d2 <- subset(dd,f=="2")

par(cex=1.5)
plot(y~x,data=d1,bg="red",pch=21,type="o")
lines(y~x,data=d2,col="blue",lwd=2)
legend("bottomright",c("group 1","group 2"),
       col=c("black","blue"),
       pch=c(21,NA),
       pt.bg=c("red",NA),
       lty=1,
       lwd=c(1,2))

(My real data are a little more complex.) I'm going a little nuts trying to do this cleanly in ggplot2 .

If I draw points before lines, group 1's points get overlaid by the lines in the same group:

 library(ggplot2); theme_set(theme_bw())
 g0 <- ggplot(dd,aes(x,y,fill=f,colour=f,shape=f))+
     scale_fill_manual(values=c("red",NA))+
     scale_colour_manual(values=c("black","blue")) +
     scale_shape_manual(values=c(21,NA))
 g0 + geom_point()+  geom_line()
 ggsave("order2.png",width=3,height=3)

If I draw lines before points, group 2's lines get overlaid by group 1's points:

 g0 + geom_line()+  geom_point()
 ggsave("order3.png",width=3,height=3)

The desired order is (group 1 lines), (group 1 points), (group 2 lines). I can do this by manually overlaying the geoms again, one group at a time, but this is way ugly.

 g0 +  geom_line() + geom_point()+ 
     geom_point(data=d1)+
     geom_line(data=d2,show.legend=FALSE)
 ggsave("order4.png",width=3,height=3)

I think the "best" solution to this is to write a low-level geom_linepoint that works as desired; I've looked into this a bit and it's not entirely trivial ... can anyone suggest a cleaner, simpler solution?

Answer 1

Here's a "low tech" ¹ solution. Below is a function that adds a line layer and then a point layer successively for each level of a given grouping variable.

linepoint = function(data, group.var, lsize=1.2, psize=4) {
  lapply(split(data, data[,group.var]), function(dg) {
    list(geom_line(data=dg, size=lsize),
         geom_point(data=dg, size=psize)) 
  })
}

ggplot(dd, aes(x,y, fill=f, colour=f,shape=f))+
  scale_fill_manual(values=c("red",NA))+
  scale_colour_manual(values=c("black","blue")) +
  scale_shape_manual(values=c(21,NA)) +
  linepoint(dd, "f")

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¹ "Low tech" compared to writing a new geom. @baptiste's (now deleted) answer does create a new geom and seems to get the job done, so I'm not sure why he deleted it.