Fill the region between two lines with ggplot2 in R

Question

This is a toy dataset:

xa <- c(4, 5, 4.5, 4, 3, 1.5)
ya <- c(1, 2, 4, 5, 5.5, 6)
xb <- c(3.8, 4.5, 4, 3.5, 2.5, 1)
yb <- c(1, 2, 3, 4, 5, 5.8)
toyset <- as.data.frame(cbind(xa, ya, xb, yb))

If we simply plot the points and lines connecting them we get:

library(ggplot2)
ggplot(toyset) + geom_path(aes(xa, ya)) + geom_path(aes(xb, yb)) +
  geom_point(aes(xa, ya)) + geom_point(aes(xb, yb))

Is there a simple way in ggplot2 to fill the region defined by the two lines?

Answer 1

You can use geom_polygon:

poly_df <- rbind(setNames(toyset[,1:2],c('x','y')),
                 setNames(toyset[6:1,3:4],c('x','y')))

ggplot(toyset) + 
    geom_path(aes(xa, ya)) + 
    geom_path(aes(xb, yb)) +
    geom_point(aes(xa, ya)) + 
    geom_point(aes(xb, yb)) +
    geom_polygon(data = poly_df,aes(x = x,y = y),fill = "lightblue",alpha = 0.25)

I fixed the typo in your sample data (54). Note that you have to be very careful about the ordering of the points in the data frame for the polygon. It's the same deal as with geom_path.