How to plot a legend with several point columns that refer to the same labels

Question

Suppose I have this example data.frame :

df = data.frame(x = c(1,1,1,2,2,2,3,3,3), y = runif(9,0,0.9))

to which I assign a colors as follows (pardon my non-sophisticated code for this part):

df$color = rep(NA, nrow(df))
color.breaks = seq(0,0.9,0.3)
colors = c("lightgray","gray","darkgray")
df$color[which(df$x == 1)] = colors[findInterval(df$y[which(df$x == 1)], color.breaks)]
colors = c("lightblue","blue","darkblue")
df$color[which(df$x == 2)] = colors[findInterval(df$y[which(df$x == 2)], color.breaks)]
colors = c("lightgreen","green","darkgreen")
df$color[which(df$x == 3)] = colors[findInterval(df$y[which(df$x == 3)], color.breaks)]

which I then plot:

plot(x = df$x, y = df$y, col = as.character(df$col), pch = 16)

Now I would like to add a legend which has three rows and four columns where the first column is the color.breaks and next three columns are the respective colors.

Something like this:

Any idea how I get this done with the legend function?

Answer 1

Here's a base solution, which need's some finer adjustment...

plot(1, type = 'n')
l <- legend(1, 1, 
       legend = rep(NA, 9), 
       ncol = 3, pch = 16, bty="n",
       col = c("lightgray","gray","darkgray", "lightblue","blue","darkblue", 
               "lightgreen","green","darkgreen"), trace = TRUE)
text(l$text$x-0.05, l$text$y, c('A', 'B', 'C', rep(NA, 6)), pos = 2)

Crucial steps are:

• Use ncol to specify the number of columns
• Omit the labels (set them to NA) and add them afterwards (to plot them to the left)
• use trace to get the positions for the labels.