Introduction
To R

Graphics

A Simple Plot

The function plot() starts a new plot. In its simplest form you give it a vector of x-coordinates and a vector of y-coordinates:

> x <- 1:20
> y <- x**3
> plot(x,y)

A new window will pop up and R will do the plot in that window. Subsequent plots will overwrite the current plot in this window.

If you want to start with a line plot instead of a point plot, use the optional argument type="l" with plot():

> plot(x,y,type="l")

Adding More

You can add points or lines to an existing plot, using the points or lines functions:

> plot(x,y)
> points(rev(x),y)
> lines(x,8000-y)

Varying Points

You can use a different character for points by using the pch= argument. Numbers give various symbols, and characters will use that letter as a marker:

> plot(x,y)
> points(rev(x),y,pch=3)   add crosses
> points(x,8000-y,pch="$") use a dollar symbol

The first 7 numeric plot symbols look like this:

Numbers 7 to 14 are composite symbols formed by overprinting these symbols, and 15 to 18 are solid-filled versions of 0 to 4.

Varying Lines

You can change line widths with lwd=, or line styles with lty=:

plot(x,y)
lines(x,y,lwd=4)        thick line
lines(rev(x),y,lty=2)   dashed line

Defining Range of Axes

If you want to fit points or lines with different ranges on the same plot, use the type="n" argument - this just sets up the margins and axes for a plot but leaves it blank. If you give it the x and y coordinates of the range of the graphs you wish to plot then the axes will be ready for adding:

> plot(c(0,20),c(-8000,8000),type='n')
> lines(x,y)
> lines(x,-y)

Adding Some Text

You can label your axes with the xlab= and ylab= functions, you can give your plot a title with the title() function, and you can put text anywhere with the text()function:

> plot(x,y,xlab="X Is Across",ylab="Y's Up")
> title("Titles are Tops")
> text(6,4000,"This goes anywhere")

Multiple Plots

You can tell the graphics device to display several small plots at a time instead of one big one. Use the par() function:

> par(mfrow=c(2,3))  2x3 array
> plot(x,y)
> plot(x,2*y)
etc

The screen clears when you try and plot the seventh graph. To reset it back to normal, do par(mfrow=c(1,1)). The par() function can also do lots of other things. More later.

Histograms

The hist() function plots a histogram of its argument.

> hist(c(2,2,2,2,2,3,3,3,4,4,5,5))

For some better examples I'll introduce you to the runif() function - this generates uniform random numbers between 0 and 1. Try this:

> x <- runif(100) 100 random numbers
> hist(x)

This shows a fairly uniform distribution of the 100 numbers. Now if we keep adding more numbers to each one, we should eventually approximate a Normal distribution. First set the graphics device to display 9 graphs at a time with par(mfrow=c(3,3)) and then repeat the following a few times:

> x <- x + runif(100)
> hist(x)

Eventually the graph should look like a Normal distribution.

Printing Plots

The plot drawn in the pop up window can now be printed out using the folllowing command:

> dev.print()
request id is hp1-16905 (standard input)
X11
2
The function returns a message indicating that the plot has been sent to the default printing device (the laser printer in the Resources Room).

Rather than sending every plot to the printer you can instead store them into files. This is particularly useful for essays and reports since these files can be easily read into standard word processors (Latex, Word, etc). To generate a Postscript file (.ps) type:

> dev.print(file="filename.ps")

Alternatively, you can type:

> postscript(file='filename.ps', hor=F)

where the option hor=F tells R that you want to create a ps file on a portrait orientation. To create a file on a landscape orientation set hor=T.
Now you will have to re-run the command(s) for creating the plot(s). For example:

> par(mfrow=c(2,1)) 2x1 array
> x <- runif(100)
> hist(x)
> x <- x + runif(100)
> hist(x)
> dev.off()
null device
1

The command dev.off() closes the ps file. You can take a look at the output file using ghostview.

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