R: change background color of plot for specific area only (based on x-values)

Question

How do I change the background color for a plot, only for a specific area? For example, from x=2 to x=4?

Bonus question: is it also possible for a combination of x and y coordinates? (for example from (1,2) to (3,4))?

Many thanks!

Answer 1

This can be achieved by thinking about the plot somewhat differently to your description. Basically, you want to draw a coloured rectangle between the desired positions on the x-axis, filling the entire y-axis limit range. This can be achieved using rect(), and note how, in the example below, I grab the user (usr) coordinates of the current plot to give me the limits on the y-axis and that we draw beyond these limits to ensure the full range is covered in the plot.

plot(1:10, 1:10, type = "n", axes = FALSE) ## no axes
lim <- par("usr")
rect(2, lim[3]-1, 4, lim[4]+1, border = "red", col = "red")
axis(1) ## add axes back
axis(2)
box()   ## and the plot frame

rect() can draw a sequence of rectangles if we provide a vector of coordinates, and it can easily handle the case for the arbitrary x,y coordinates of your bonus, but for the latter it is easier to avoid mistakes if you start with a vector of X coordinates and another for the Y coordinates as below:

X <- c(1,3)
Y <- c(2,4)
plot(1:10, 1:10, type = "n", axes = FALSE) ## no axes
lim <- par("usr")
rect(X[1], Y[1], X[2], Y[2], border = "red", col = "red")
axis(1) ## add axes back
axis(2)
box()   ## and the plot frame

You could just as easily have the data as you have it in the bonus:

botleft <- c(1,2)
topright <- c(3,4)
plot(1:10, 1:10, type = "n", axes = FALSE) ## no axes
lim <- par("usr")
rect(botleft[1], botleft[2], topright[1], topright[2], border = "red",
     col = "red")
axis(1) ## add axes back
axis(2)
box()   ## and the plot frame