How to make R legend with 2 columns?

Question

I want to make a legend on my graph, which is generated by plot() function. The original legend() function will generate a list which has only 1 column. How can I make a legend which has 2 columns?

Answer 1

I could not find a way to do that within a single call to legend for standard plots.

Here's an option, drawing two separate legends: one with lines and points, one with labels. x.intersp can be used to tweak distance between labels and lines.

plot(cumsum(runif(n = 100)))

# draw legend with lines and point but without labels and box. x.intersp controls horizontal distance between lines
L = legend(x = 'bottom', legend = rep(NA,4), col=1:2, lty=c(1,1,2,2), ncol=2, bty='n', x.intersp=0.5, pch=c(1,2,1,2), inset=0.02)

# use position data of previous legend to draw legend with invisble lines and points but with labels and box. x.intersp controls distance between lines and labels
legend(x = L$rect$left, y = L$rect$top, legend = c('Group A', 'Group B'), col=rep(NA,2), lty=c(1,1), ncol=1, x.intersp = 3, bg = NA)

Answer 2

Check this:

library(lattice)

myPCH <- 15:17
Data  <- rnorm(50)
Index <- seq(length(Data))

xyplot(Data ~ Index, 
       pch = myPCH, col=1:2,
       key = list(space = "right", adj=1,
                  text = list(c("a", "b", "c"), cex=1.5),
                  points = list(pch = myPCH),
                  points = list(pch = myPCH,col=2)))