Drawing polygonal chains in ggplot

Question

How to draw multiples polygonal chains in ggplot? I was writing something like:

x=c(1,3,4,5,6)
y=c(0.5,2,3,7,1)
z=c(8,2,6,7,8)
n=length(x)-1
library(ggplot2)
p <- ggplot(mtcars, aes(wt, mpg))
for (i in 1:n){
  p <-p + geom_segment(aes(x = x[i], y = y[i], xend = x[i+1], yend = y[i+1]), colour = "red")
  p$plot_env <- list2env(list(x=x,y=y))
  }

Drawing two polygonal chains:

n=length(x)
pol1=c(x,y)
pol2=c(y,z)
g=c(rep(1,n),rep(2,n))
library(ggplot2)
p0 <- ggplot(mtcars,aes(wt,mpg))
p0 + geom_path(aes(pol1,pol2,group=g),data=data.frame(pol1,pol2))

Answer 1

I can't quite tell what you want here.

• Are you specifying more than one path? I only see a single set of x and y values.
• What is z supposed to be doing?

You can draw a single (unclosed) path via

p0 <- ggplot(mtcars,aes(wt,mpg))
p <- p0 + geom_path(aes(x,y),data=data.frame(x,y))

Or a closed path via

p0 + geom_polygon(aes(x,y),data=data.frame(x,y),fill=NA,
             colour="black")

If you do have more than one path, you can combine all the vertices in two long vectors x and y, make up a g vector to distinguish paths, and use aes(x,y,group=g) to get the paths/polygons drawn separately.