Basic Graphics

S has many powerful graphics functions. In order to take advantage of them on a unix machine, S requires that you first specify the type of graphics device you are using. R users on the Mac can skip down to the list of graphics functions at the bottom of this page.

> motif()         # open a graphics window 
> postscript()  # send graphics output to a postscript printer
> printer()     # produce line printer plots: necessary if you are at a 
                # `dumb' terminal, but very low resolution

Usually you will want to use motif(); it is easy to print directly from the graphics window to a laserprinter. To print graphics directly from the graphics window, you need to do two things:

• First make sure a printer is selected: (pull down the options menu at the top of the print window, and select printing, then in the box labeled `Command', be sure a postscript printer is selected. It should look like
  
  lpr -r -h -Pdrafts

  Click on the save button, and the print command will be permanently recorded so that you don't have to do this every time you want to print a graph in S; click on the apply button to save the print command for the duration of your current S session only.

• To print, pull down the Graph menu and select print.

Basic S graphics functions include:

Function Name          Description 

hist(x)             draw a histogram of the data in x 
stem(x)	            draw a stem and leaf diagram of the data in x
boxplot(x)          draw a box plot of x 
plot(x,y)           draw a scatterplot of y on x
contour(x,y,z)      draw a contour plot; z[i,j] is the height
                          of the surface at (x[i],y[j])

lines(x,y)          add a line to an existing plot, connectin successive
			pairs of points (x[i],y[i])
text(x,y,z)         plot symbol z[i] at coordinates (x[i],y[i])
title("My Title")   add a title to the current plot \\
legend(...)         add a legend to a plot

S has very sophisticated graphics capabilities which allow the user to add lines, text, and so forth to existing plots, highlight selected points, plot subsets of the data using different plot characters, and so on. The following example shows how to make a scatterplot of two variables, x and y, using a third variable, grp, to determine the plot character.

Example

> motif()                  # open a graphics window
> grp <- rep(c(1,2),25)      # make up group numbers
> x <- rnorm(50) + grp       # generate x data
> y <- rnorm(50) + grp       # generate y data
> plot(x,y,type="n")       # set up plot region
> text(x,y,grp)              # plot group numbers
> legend(locator(1), legend=c("Group 1","Group 2"),pch=c("12"))
                           # add legend to the plot: click the left
			   # mouse button in a blank area

>     #alternatively you could specify plot symbols by groups:
> plot(x,y,type="n")
> points(x[grp==1],y[grp==1],pch="o")
>            # plot the x,y points from group 1 with an "o"
> points(x[grp==2],y[grp==2],pch="x")
>            # plot group 2 with "x"'s