ggplot2: use geom_line() carefully when your x-axis data are descending

Given 3 points, $(1,1), (2,1), (2,2)$, how would you connect them with 2 segments? Subtly, geom_line of ggplot2 thinks in a different way when you order the points by $x$ descendingly.

library(ggplot2)
library(cowplot)

df <- data.frame(x=c(2,2,1), y=c(2,1,1))

p_line <- ggplot(data=df, mapping=aes(x=x, y=y)) + 
	      geom_line(size=0.3) + geom_point(size=0.4, color=I("blue")) + 
	      ggtitle("geom_line")

p_path <- ggplot(data=df, mapping=aes(x=x, y=y)) + 
	      geom_path(size=0.3) + geom_point(size=0.4, color=I("blue")) + 
	      ggtitle("geom_path")

df_rev <- data.frame(x=rev(c(2,2,1)), y=rev(c(2,1,1)))

p_line_rev <- ggplot(data=df_rev, mapping=aes(x=x, y=y)) + 
	          geom_line(size=0.3) + geom_point(size=0.4, color=I("blue")) + 
	          ggtitle("geom_line + rev")

p_path_rev <- ggplot(data=df_rev, mapping=aes(x=x, y=y)) + 
	          geom_path(size=0.3) + geom_point(size=0.4, color=I("blue")) + 
	          ggtitle("geom_path + rev")

plot_grid(p_line, p_path, p_line_rev, p_path_rev)

geom_line connects dots on the wrong axis mentioned that:

geom_line joins lines up from the minimum x to maximum

Obviously in my example, geom_line did not follow the order in df_rev, i.e. $(1,1) \rightarrow (2,1) \rightarrow (2,2)$. geom_line would not sort tied x’s by y, but leave them in the original order, so it became $(1,1) \rightarrow (2,2) \rightarrow (2,1)$.

If you are certain that your x is in order and want to connect them in that very order, just use geom_path.