library(ggplot2)

# to plot a rectangle
x = c(0, 1, 1, 0, 0)
y = c(0, 0, 1, 1, 0)
ggplot(NULL, aes(x, y))   geom_line()

CodePudding user response：

The reason that the top line is missing is that geom_line sorts the co-ordinates according to their position on the x axis before plotting them. It does not plot points in the order in which they appear in the vector.

It is a geom_path that allows arbitrary ordering of x and y values and would produce the lines you want:

x = c(0, 1, 1, 0, 0)
y = c(0, 0, 1, 1, 0)
ggplot(NULL, aes(x, y))   geom_path()

In a sense, a geom_line is just a geom_path where the points have been ordered by ascending x value. We can replicate the behaviour of geom_line by ordering manually:

ggplot(NULL, aes(x[order(x)], y[order(x)]))   geom_path()