An xpd-tion into R plot margins

November 20, 2014
By

(This article was first published on Fellgernon Bit - rstats, and kindly contributed to R-bloggers)

This is a guest post by Prasad Patil that answers the question: how to put a shape in the margin of an R plot?

The help page for R‘s par() function is a somewhat impenetrable list
of abbreviations that allow you to manipulate anything and everything
in the plotting device. You may have used this function in the past
to create an array of plots (using mfrow or mfcol) or to set margins
(mar or mai).

Way down toward the end of the list is the often-overlooked xpd parameter.
This value specifies where in the plotting device an object can actually
be plotted. The default is xpd = FALSE, which means that plotting is clipped,
or restricted, to the plotting region. In other words, if your plot has
xlim = c(0, 10) and ylim = c(0, 10) and you try to plot the point (-1, -1), it will
not appear anywhere in the device.

xpd takes two other values, TRUE and NA, which limit plotting to the figure
and device region, respectively. If you’re fuzzy on plotting terms, this
tutorial

presents those topics well.

Plotting outside the plot

If you want to plot outside of the plotting region, I find that setting xpd = NA
easiest since it opens up all external space. We also need to make sure that we
keep space outside of the plot so that we have room to place our objects. Let’s
say we want to put an ugly border above and below our plot:

# Set xpd=NA and expand the top and bottom margins
par(xpd = NA, mar = par()$mar + c(2.5, 0, 1, 0))
plot(1:10)
# Note that the rectangle we make here has corner coordinates outside of
# our plotting device
rect(-5, 11, 12, 14, col="red")
# Random dots in our rectangluar region
points(runif(100, -4.2, 12.8), runif(100, 11.2, 13.6), col = "green", pch = 19, cex = 1.2)
# And another rectangle for below
rect(-5, -1.7, 12, -3.5, col="red")
points(runif(100, -4.2, 12.8), runif(100, -3.3, -1.8), col = "green", pch = 19, cex = 1.2)

center

Here we mentally extend the axes of our plot to determine where to put
our margin elements. One can imagine a diagonal for the top rectangle
running from (-5,11) to (12,14). Neither of these points appear in the plot
itself, but we used the established axes to estimate them and plot outside
the plotting region.

Images outside the plot

Now let’s say we want to add a logo or other external image in the margin
of our plot. We will use R‘s png library to load a PNG image and
rasterImage() to plot it:

## If needed: install.packages("png")
library(png)
img <- readPNG("logo.png")
par(xpd = NA, mar=par()$mar + c(3, 0, 0, 0))
plot(1:10)
rasterImage(img, 0.5, -2.5, 10.5, -1)

center

Here we used the png library and the rasterImage() command to read in and
plot the “logo.png” file. Based on the previously-known dimensions of the
logo, we can choose which points to use as endpoints for the image. Note
that this image may appear stretched or contorted depending on the size
of your R plot device, and it will not stay consistent if you resize.

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