Plotting Data with Gnuplot

Gnuplot is an all-purpose, interactive plotting program that is available for Linux and Macintosh operating systems. This page provides a brief primer on how to plot your data files with Gnuplot. It is not intended to be a comprehensive description of the Gnuplot software package, and in particular contains no information about installing Gnuplot on your machine. If you have outstanding questions, or would like to learn more features of the software, please refer to the Gnuplot Manual, or consult one of the many excellent tutorials available on the web. Some recommended Gnuplot sites are listed in the References section of this document.

Uncompressing Your Data Files

The data files available for you to download will be in gzipped .csv format. To make these suitable for using with Gnuplot, you should:

• gunzip the files, i.e. at the command prompt, type:
  gunzip data*.gz

Getting Started

Gnuplot allows you to create many different types of plots for your data. In this document, we only describe how to create a very basic plot - we encourage you to experiment, and to find more sophisticated ways of displaying your data.

• To launch gnuplot in interactive mode, type the following command:
  gnuplot
  The program will launch and display the gnuplot prompt:
  gnuplot>
• To launch gnuplot in batch mode, type the following command:
  gnuplot inputfile
  where inputfile is the name of a file containing gnuplot commands (also called a load file). More than one load file can be specified; in this case, the syntax is:
  gnuplot file1 file2 file3 etc.
  
• To terminate the gnuplot program, type exit or quit.

Basic Gnuplot Commands

In this section, we describe some basic gnuplot commands that you will use over and over again. To learn more gnuplot commands or advanced options of the commands listed here, you should consult the Gnuplot Manual.

• plot
  plot is the primary command for drawing plots with gnuplot. It creates 2-D plots of functions and data in many, many ways. Here are some examples of how the plot command is used:
  plot "data000001.csv" using 1:2 with lines
  This plots the first two columns of the datafile data000001.csv using l ines to connect the data points.

  plot "data000001.csv" using 4:6 with points, "data000002.csv" using 4:6 with points
  This plots the fourth (x-axis) and sixth (y-axis) columns of the datafiles data000001.csv and data000002.csv using large dots to represent the data points. The two datafiles will be represented in different colours. (Actually, the expression with points can be omitted since it is the default value.)

  plot [0:100] [10:20] "data000001.csv" using 1:2 title "Distance vs Time" with dots
  This plots the first two columns of the datafile data000001.csv within the x-range (0,100) and y-range (10,20) using small dots to represent the data points. "Distance vs Time" appears as the title on the plot.

• set and show
  set is used to specify many different plotting options. show allows the user to view these options. Note that no plot is drawn, however, until a plot command is given. Here are some examples of how the set command is used (the show command uses identical syntax):
  set title "My Plot"
  This command centres the words "My Plot" at the top of the plot. set notitle clears the title.

  set origin 0.1, 0.1
  This command places the origin of the plot at screen position (0.1,0.1), where (0,0) specifies the bottom left corner of the window, and (1,1) specifies the top right corner of the window.

  set xlabel "time"
  This command labels the x axis "time". The equivalent set command for the y axis is set ylabel "time"

  set xrange [0:100]
  This command limits the x axis range of the plot to (0,100). The equivalent set command for the y axis is set yrange [0:100]

  set logscale x
  This command enables logscaling on the x axis. set logscale y is the equivalent command for the y axis, and set nologscale disables logscaling.

  set terminal gif
  This command enables the user to make a GIF version of their plot, by redirecting the subsequent plotting commands to a driver that makes a GIF file. You would use the set terminal command for generating your plot in a format that can be printed out or displayed on the web (for example). Many different terminal settings are supported (refer to the Gnuplot manual for more details). Other set terminal commands you might find useful include set terminal postscript and set terminal png (Portable Network Graphics format).

• load
  load allows the user to write scripts of commonly used Gnuplot commands, rather than having to re-enter each command in interactive mode. Each line of the specified input file (in the example, gnuplot.in) is executed as if it had been typed in interactively. Usage:
  load "gnuplot.in"

• pause
  The pause command displays any text associated with the command and then waits a specified amount of time or until the carriage return is pressed. pause is especially useful in conjunction with load files. For example, the following command writes "This is a quadratic (Hit return to continue)" to the screen, and then waits 5 seconds (or until return is pressed):
  pause 5 "This is a quadratic (Hit return to continue)"

Gnuplot References

The Gnuplot manual at http://www.ucc.ie/gnuplot/gnuplot.html should provide the answer to most questions. However, you might also like to refer to the following Gnuplot references on the web:

• Gnuplot Central (Official Gnuplot Page, includes installation, mirror and ftp information)
  http://www.gnuplot.info/
• Gnuplot FAQ newsgroup
  http://www.uni-karlsruhe.de/~ig25/gnuplot-faq/
• University of Northern Iowa Gnuplot Help Page
  http://www.cs.uni.edu/Help/gnuplot/
• Duke University Gnuplot Tutorial
  http://www.duke.edu/~hpgavin/gnuplot.html
• Gnuplot for Macintosh (please note that the SAOteam has not tested Gnuplot versions under operating systems other than Linux... use at your own risk!)
  http://homepage.mac.com/gnuplot/
• Gnuplot for Windows (please note that the SAOteam has not tested Gnuplot versions under operating systems other than Linux... use at your own risk!)
  http://archives.math.utk.edu/software/multi-platform/gnuplot/msdos/
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© Swinburne Copyright and disclaimer information
Maintained by: Rebecca Allen (rebeccaallen@swin.edu.au)
Authorised by: Prof. Jean Brodie (jbrodie@swin.edu.au)
Monday, 19-Nov-2007 11:20:02 AEDT