Easy labels for multi-panel plots in R

There are a number of ways to make multi-panel figures in R.  Probably the easiest and most commonly used method is to set par(mfrow=c(r,c)) to the  number of rows (r) and columns (c) you would like to use for your figure panels (Note: par(mfcol=c(r,c)) produces the same thing, only it renders the figures by column rather than by row).  Other methods include par(fig=c(x1,x2,y1,y2), new=T) and layout(mat), but these will be for another post.

What I found challenging was putting a label in a consistent location on each of the panels.  Using the text() function would be the go-to function for this, but the default coordinate system used in the text() is the plot’s coordinate system, and you’ll have to set an additional plotting option (par(xpd)) to plot outside of the figure region.

Since I regularly make multi-panel figures, I decided to write a wrapper function around text() that can easily and consistently place a label on a generated plot without having to worry about plotting coordinates.  Below is the function (Note: You can find the code and an example of its usage on bitbucket https://bitbucket.org/ggg121/r_figure_letter.git)

put.fig.letter <- function(label, location="topleft", x=NULL, y=NULL, 
                           offset=c(0, 0), ...) {
  if(length(label) > 1) {
    warning("length(label) > 1, using label[1]")
  }
  if(is.null(x) | is.null(y)) {
    coords <- switch(location,
                     topleft = c(0.015,0.98),
                     topcenter = c(0.5525,0.98),
                     topright = c(0.985, 0.98),
                     bottomleft = c(0.015, 0.02), 
                     bottomcenter = c(0.5525, 0.02), 
                     bottomright = c(0.985, 0.02),
                     c(0.015, 0.98) )
  } else {
    coords <- c(x,y)
  }
  this.x <- grconvertX(coords[1] + offset[1], from="nfc", to="user")
  this.y <- grconvertY(coords[2] + offset[2], from="nfc", to="user")
  text(labels=label[1], x=this.x, y=this.y, xpd=T, ...)
}

Simply source the code at the start of your R script and you’re good to go.  For convenience, the wrapper function allows you to use text-based locations for common figure-label locations akin to the keyword functionality in the legend() function.  Also, additional parameters can be passed to text() through this function.  Below is an example.

# Source the legend function file
source("put_fig_letter.r")

# Set the seed
set.seed(1234)

# Generate a data point to plot
x <- matrix(rnorm(60), ncol=6)
y <- matrix(rnorm(60), ncol=6)

# Apply a random scale to each column
x <- apply(x, 2, function(x) x*runif(1)*10)
y <- apply(y, 2, function(x) x*runif(1)*10)

# Setup multiple plot regions
par(mfrow=c(2,3), mar=c(5,4,1.5,1)+0.1)

# You can feed an (x,y) location to put the figure
# letter if you like, or you can use a predefined
# location by name kind of like legend()
my.locations <- c("topleft", "topcenter", "topright",
            "bottomleft", "bottomcenter", "bottomright")

# Make the plots and append a figure letter to each
# Note: put.fig.letter() sends additional parameters to
# the text() function.
for(i in 1:6) {
  plot(x[,i], y[,i], pch=16, xlab="x", ylab="y")
  my.label <- paste(letters[i], ".", sep="")
  put.fig.letter(label=my.label, location=my.locations[i], font=2)
}

And here’s the resulting figure: