Axes and labels in R

Many high level plotting functions (plot, hist, boxplot, etc.) allow you to include axis and text options (as well as other graphical parameters). For example:

 # Specify axis options within plot()
plot(x, y, main="title", sub="subtitle",
  xlab="X-axis label", ylab="y-axix label",
  xlim=c(xmin, xmax), ylim=c(ymin, ymax))

For finer control or for modularization, you can use the functions described below.

Titles

Use the title( ) function to add labels to a plot.

title(main="main title", sub="sub-title",
 xlab="x-axis label", ylab="y-axis label")

Many other graphical parameters (such as text size, font, rotation, and color) can also be specified in the title( ) function.

# Add a red title and a blue subtitle. Make x and y
# labels 25% smaller than the default and green.
title(main="My Title", col.main="red",
  sub="My Sub-title", col.sub="blue",
  xlab="My X label", ylab="My Y label",
  col.lab="green", cex.lab=0.75)

Text Annotations

Text can be added to graphs using the text( ) and mtext( ) functions. text( ) places text within the graph while mtext( ) places text in one of the four margins.

text(location, "text to place", pos, ...)
mtext("text to place", side, line=n, ...)

Common options are described below.

Other common options are cex , col, and font (for size, color, and font style respectively).

Labeling points

You can use the text( ) function (see above) for labeling point as well as for adding other text annotations. Specify location as a set of x, y coordinates and specify the text to place as a vector of labels. The x, y, and label vectors should all be the same length.

# Example of labeling points
attach(mtcars)
plot(wt, mpg, main="Milage vs. Car Weight",
  xlab="Weight", ylab="Mileage", pch=18, col="blue")
text(wt, mpg, row.names(mtcars), cex=0.6, pos=4, col="red")

Math Annotations

You can add mathematically formulas to a graph using TEX-like rules. See help(plotmath) for details and examples.

Axes

You can create custom axes using the axis( ) function.

axis(side, at=, labels=, pos=, lty=, col=, las=, tck=, ...)

where

If you are going to create a custom axis, you should suppress the axis automatically generated by your high level plotting function. The option axes=FALSE suppresses both x and y axes. xaxt="n" and yaxt="n" suppress the x and y axis respectively. Here is a (somewhat overblown) example.

# A Silly Axis Example

# specify the data
x <- c(1:10); y <- x; z <- 10/x

# create extra margin room on the right for an axis
par(mar=c(5, 4, 4, 8) + 0.1)

# plot x vs. y
plot(x, y,type="b", pch=21, col="red",
  yaxt="n", lty=3, xlab="", ylab="")

# add x vs. 1/x
lines(x, z, type="b", pch=22, col="blue", lty=2)

# draw an axis on the left
axis(2, at=x,labels=x, col.axis="red", las=2)

# draw an axis on the right, with smaller text and ticks
axis(4, at=z,labels=round(z,digits=2),
  col.axis="blue", las=2, cex.axis=0.7, tck=-.01)

# add a title for the right axis
mtext("y=1/x", side=4, line=3, cex.lab=1,las=2, col="blue")

# add a main title and bottom and left axis labels
title("An Example of Creative Axes", xlab="X values",
  ylab="Y=X")

Minor Tick Marks

The minor.tick( ) function in the Hmisc package adds minor tick marks.

# Add minor tick marks
library(Hmisc)
minor.tick(nx=n, ny=n, tick.ratio=n)

nx is the number of minor tick marks to place between x-axis major tick marks. ny does the same for the y-axis. tick.ratio is the size of the minor tick mark relative to the major tick mark. The length of the major tick mark is retrieved from par("tck").

Reference Lines

Add reference lines to a graph using the abline( ) function.

abline(h=yvalues, v=xvalues)

Other graphical parameters (such as line type, color, and width) can also be specified in the abline( ) function.

# add solid horizontal lines at y=1,5,7
abline(h=c(1,5,7))
# add dashed blue verical lines at x = 1,3,5,7,9
abline(v=seq(1,10,2),lty=2,col="blue")

Note: You can also use the grid( ) function to add reference lines.

Legend

Add a legend with the legend() function.

legend(location, title, legend, ...)

Common options are described below.

Other common legend options include bty for box type, bg for background color, cex for size, and text.col for text color. Setting horiz=TRUE sets the legend horizontally rather than vertically.

# Legend Example
attach(mtcars)
boxplot(mpg~cyl, main="Milage by Car Weight",
  yaxt="n", xlab="Milage", horizontal=TRUE,
  col=terrain.colors(3))
legend("topright", inset=.05, title="Number of Cylinders",
  c("4","6","8"), fill=terrain.colors(3), horiz=TRUE)

For more on legends, see help(legend). The examples in the help are particularly informative.

To Practice

Try the free first chapter of this online data visualization course in R.

This content is taken from statmethods.net.