shade regions in R plot - r

Question

I have a plot in R to analyse the functional behaviour of this function :
-2.5*log10(x) in different regions , the inputs are like this
curve(-2.5*log10(x),0,5,main="Behaviour of magnification function",col=2)
abline(v=0,col=3)
abline(v=1,col=4)
abline(h=0,col=8)
Now I would like to shade those regions on the right of the v=1 line and left with two different shades, but can't figure it out how to do. the polygon function not giving the shade , please help
thanks,
Ayan

Answer 1

You can use the panel.first argument to draw in the background.
Define a function to do the drawing:
fnShade = function(v,colL,colR) {
p = par("usr") # limits of the plot
rect(p[1],p[3],v,p[4],col=colL,border=NA)
rect(v,p[3],p[2],p[4],col=colR,border=NA)
abline(v=v,col=4)
}
Then use it
curve(-2.5*log10(x),0,5,main="Behaviour of magnification function",col=2,
panel.first=fnShade(1,5,"grey80"))
You can add your other ablines into the function if you like. And it should be easy to extend to draw 4 rectangles - one for each quardant - if desired.

Answer 2

Here is a ggplot solution to the problem, using annotate:
First, save the results from curve in df:
df <- curve(-2.5*log10(x),0,5,main="Behaviour of magnification function",col=2)
Then use geom_vline to add the vertical lines, and annotate to add a rect geom to the data for the shaded areas. We use annotate, because we do not want to use data from the original data frame as suggested in this answer.
plot <- ggplot(data.frame(df)) +
geom_line(aes(x, y), size = 1) +
geom_vline(xintercept = 1, color = "blue", size = 1) +
geom_hline(yintercept = 0, color ="grey", size = 1) +
geom_vline(xintercept = 0, color = "green", size = 1) +
annotate("rect", xmin = 1, xmax = Inf, ymax = Inf, ymin = -Inf, fill = "blue", alpha = 0.2) +
annotate("rect", xmin = -Inf, xmax = 1, ymax = Inf, ymin = -Inf, fill = "green", alpha = 0.2) +
labs(title = "Behaviour of magnification function\n") +
theme_bw()
plot