We will use the base graphics library for this recipe, so all you need to do is run the recipe at the R prompt. It is good practice to save your code as a script to use again later.

Let's draw a vertical line at the month of September in the rainfall graph for Tokyo:

rain <- read.csv("cityrain.csv")
plot(rain$Tokyo,type="b",lwd=2,
xaxt="n",ylim=c(0,300),col="black",
xlab="Month",ylab="Rainfall (mm)",
main="Monthly Rainfall in Tokyo")
axis(1,at=1:length(rain$Month),labels=rain$Month)

abline(v=9)

To draw marker lines with abline() at specific X or Y locations, we have to set the v (as in vertical) or h (as in horizontal) arguments respectively. In the example, we set v=9 (the index of the month September in the Month vector).

Now let's add a red dotted horizontal line to the graph to denote a high rainfall cutoff of 150 mm:

abline(h=150,col="red",lty=2)