TweetFollow Us on Twitter

An Introduction to Gnuplot

Volume Number: 26
Issue Number: 02
Column Tag: math

An Introduction to Gnuplot

what Gnuplot is and how to use it

by Mihalis Tsoukalos

Introduction

This article is going to present to you a very useful tool called Gnuplot. Gnuplot is copyrighted but freely distributed. There are versions of Gnuplot for UNIX, IBM OS/2, MS Windows, DOS, Macintosh, VMS, Atari and many other platforms.

There is also a Python module for using Gnuplot inside of Python code. The library is called py-gnuplot, and will be introduced in this article.

What is Gnuplot?

Gnuplot is a command line tool that was initially developed in order to allow scientists and students to visualize mathematical functions and data. It now supports web scripting and integration as a plotting engine for third-party applications like Octave.

Gnuplot can draw many types of plots in either 2D or 3D. It supports drawing using lines, points, vector fields, surfaces, boxes, and contours. You can also add text in its output.

It also supports output in many different file formats including eps, fig, jpeg, LaTeX, METAFONT (invented by Donald Knuth the creator of TeX), pdf, png, postscript, etc.

I will have to tell you that if you want to become an advanced Gnuplot user, you must start reading its official documentation. In my system, the gnuplot.pdf file (the official manual) is located inside the /opt/local/share/doc/gnuplot directory, but location will depend on your installation method.

Some simple examples

First of all, let me review the version of Gnuplot that I am using. You can find that info by running the following command from Terminal.app (or a similar application):

$ gnuplot -V
gnuplot 4.2 patchlevel 5

An analogous command will let you you find out the Python version that is installed into your Mac-Python will be used later on in this article. In my system, I had the following output:

$ python -version
Python 2.5.2

The first example that I will demonstrate you,will draw the sin(x)*cos(x) function. You can do this by running the following command in the gnuplot command line. This can be done after running gnuplot from a Terminal.app window:

gnuplot> plot sin(x)*cos(x)

After pressing the return key, an AquaTerm output window will appear (AquaTerm was likely installed by your port manager, like MacPorts. You can retrieve it manually from http://aquaterm.sourceforge.net/). You can see the generated output in Figure 1.


Figure 1: drawing the sin(x)*cos(x) function

Now, we will try to beautify the output-that is pretty basic and simplistic-and maybe add some text to it.

First, if you remember your Math classes, it doesn't make sense to use -10 and +10 as our graph boundaries when dealing with trigonometric functions-it would be better to use -2*pi and +2*pi instead. To do this, the following two commands must be given:

gnuplot> set xrange [-2*pi:2*pi]
gnuplot> replot

As you can see, the "set xrange" command defines the graph boundaries, but the "replot" command is needed for Gnuplot to sketch the new output.

Please note that the AquaTerm output is updated automatically, so please do not close the AquaTerm application-this will make your life a lot easier. Also note that the numerical value of pi is proximately 3.14. The new output can be seen in Figure 2.


Figure 2: using -2*pi and +2*pi as our x-axis boundaries

The next thing that we would like to do is add our own text to the output. Figure 3 shows the output that is produced by executing the following gnuplot commands:

gnuplot> set title "An example for MacTech"
gnuplot> set xlabel "x-axis: from -2*pi to +2*pi"
gnuplot> set ylabel "Setting the y-axis label"
gnuplot> replot


Figure 3: adding some text to the output

Now, grid lines are going to be added to our output (you can see the produced output in Figure 4). The following gnuplot commands must be given:

gnuplot> set xtics ("0" 0, "-180" -pi, "-90" -pi/2, "90" pi/2, "180" pi)
gnuplot> set ytics ("0" 0, "0.5" 0.5, "-0.5" -0.5) 
gnuplot> set grid
gnuplot> replot


Figure 4: adding gridlines to the gnuplot output

More advanced examples

This part of the article will present you with some more advanced ways of using gnuplot. After all, gnuplot is a scientific tool.

Plotting more than one function in the same output

Let's say that you want to see two functions in the same output in order to compare them. Gnuplot can help you do this. The following command explain how:

gnuplot> plot x*x, 1/(x*x)

As you see, all you have to do is separate the two functions with a comma. If you want to smarten the output, you can give the following command:

gnuplot> plot x*x, 1/(x*x) with filledcurves fs 0

You can see the final output in Figure 5 and you can easily understand that the "with filledcurves fs 0" does all the work for us-it fills the second function with gray color. Please note that gnuplot has a built-in help system. You can say, for example, help plot and get useful information about the plot command-just try it!


Figure 5: plotting two functions

Plotting many functions in multiplot mode

Sometimes it is preferred to show multiple graphs side-by-side. Gnuplot gives you the choice of plotting multiple graphs side-by-side on the same output screen. The key point for doing this is the "set multiplot" command. If you give this command the prompt will change from "gnuplot" to "multiplot".

You can see the output of the following example in Figure 6:

gnuplot> set xrange [-1:1]
gnuplot> set size 1,1
gnuplot> set origin 0,0
gnuplot> set multiplot
multiplot> set size 0.5,0.5
multiplot> set origin 0,0.5
multiplot> plot (x*x)
multiplot> set size 0.5,0.5
multiplot> set origin 0.5,0.5
multiplot> plot sin(x)
multiplot> set size 0.5,0.5
multiplot> set origin 0,0    
multiplot> plot (1/x)
multiplot> set size 0.5,0.5
multiplot> set origin 0.5,0
multiplot> plot (x*x*x)
multiplot> unset multiplot
gnuplot> reset
gnuplot>


Figure 6: using multiplot mode

Now, lets talk about the commands that were typed in order to get the output of Figure 6. The plot and xrange commands were already shown earlier. Also, as we said before, the "set multiplot" command is needed in order to enter the multiplot mode. Similarly, the "unset multiplot" command is used for exiting multiplot mode. The reset command causes all graph-related options that can be set with the set command to take on their default values. The rest of the commands (size, origin) position each graph in its place and make it take just a quarter of the screen. Again, if you want more information about a command you should see the help pages.

Plotting data files

Imagine that you have a file that contains groups of data points that you want to plot. Well, gnuplot can help you again! The following commands illustrate what gnuplot shows when plotting a valid data file-nothing, it just plots it!- as well as when trying to parse an erroneous data file. For an erroneous data file, gnuplot prints an error message telling us the line that the first error occurred-this does not indicate that the rest of the file is fine, just that there is at least one error. The compatible data file gets plotted without problems and you can see its output in Figure 7.

gnuplot> plot 'dataBad'
              ^
         Bad data on line 325
gnuplot> plot 'data'


Figure 7: using external data files when drawing a graph

The format of the "data" file is as follows but many other formats are allowed:

$ head data
1024   38
1025   5
1026   18
1027   16
1028   20
1029   6
1038   5
1048   4
1049   16
1051   4

Using Gnuplot and Python

This part of the article will present py-gnuplot-the Python interface to the gnuplot plotting program.

The first thing you should do is to install py-gnuplot. I used MacPorts (there is also a fink version of the package) version of py-gnuplot-the simple installation process was as follows:

$ sudo port install py25-gnuplot
-->  Fetching python25
-->  Attempting to fetch Python-2.5.2.tgz from http://www.python.org//ftp/python/2.5.2/
-->  Verifying checksum(s) for python25
-->  Extracting python25
-->  Applying patches to python25
-->  Configuring python25
-->  Building python25 with target all libpython2.5.dylib
-->  Staging python25 into destroot
-->  Installing python25 2.5.2_5+darwin_9
-->  Activating python25 2.5.2_5+darwin_9
[output removed for brevity]
-->  Staging py25-gnuplot into destroot
-->  Installing py25-gnuplot 1.7_0
-->  Activating py25-gnuplot 1.7_0
-->  Cleaning py25-gnuplot

So, we now have both Python 2.5 and py-gnuplot 1.7 installed. It now is time to start using them.

I would first like to tell you that in this part of the article, the output of each Python script is going to be saved in a file and will not be displayed on screen. I found this the most practical usage for a script.

A simple example is shown in Listing 1:

Listing 1: gp-example.py

#! /usr/bin/env python
# Programmer: Mihalis Tsoukalos
# Date: Friday 10 July 2009
#
# Description: Example of using py-gnuplot
#
import Gnuplot
gp = Gnuplot.Gnuplot(persist=1)
gp('set term png')
gp('set output "mactech.png"')
g5 = Gnuplot.Func('sin(x)/x', title='Plotting a function with py-gnuplot')
gp.plot(g5)

The output, which is a file called mactech.png, is shown in Figure 8. The key points of the Python script are:

The "import Gnuplot" command that load the correct Python module

The "gp.plot(g5)" command that actually plots the graph

The "gp('set term png')" and "gp('set output "mactech.png"')" commands that define the output file format and filename.


Figure 8: plotting a function using Python and py-gnuplot

Please note that you should read the py-gnuplot documentation in order to better understand and utilize it.

The last example of this article is going to be a 3D graph. The Python code for creating Figure 9 is the following:

#! /usr/bin/env python
# Programmer: Mihalis Tsoukalos
# Date: Friday 10 July 2009
#
# Description: Example of using py-gnuplot
#
# This script is a modified version of existing Python code
# from the demo.py file that can be found inside
# the /opt/local/lib/python2.5/site-packages/Gnuplot directory
#
from Numeric import *
import Gnuplot, Gnuplot.funcutils
gp = Gnuplot.Gnuplot(persist=1)
gp('set term png')
gp('set output "3Dplot.png"')
# set up x and y values at which the function will be tabulated:
x = arange(50)/2.0
y = arange(25)/10.0 - 1.5
xm = x[:,NewAxis]
ym = y[NewAxis,:]
m = (cos(xm) + 0.1*xm) - ym**2
gp('set parametric')
gp('set data style lines')
gp.title('An example of a 3D plot!')
gp.xlabel('x')
gp.splot(Gnuplot.GridData(m,x,y, binary=0))

You may find it difficult but, after reading it more carefully, you will understand how simple and elegant this is.


Figure 9: doing 3D plots with py-gnuplot

Summary

Gnuplot is a very handy and highly customizable tool. This article is just the beginning of learning it. The excellent on-line help of gnuplot can assist you go further.

By combining it with Python-or your favorite scripting language-you will get the benefits of a scripting language in conjunction with a great tool!

As always, the key to success is practicing, more practicing and even more practicing!

Web Links and Bibliography

Gnuplot homepage: http://www.gnuplot.info/

Gnuplot development: http://sourceforge.net/projects/gnuplot/

Gnuplot Python package: http://gnuplot-py.sourceforge.net/


Mihalis Tsoukalos lives in Greece with his wife Eugenia and enjoys digital photography and writing articles. He is the author of the “Programming Dashboard Widgets” eBook. You can reach him at tsoukalos@sch.gr.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

OmniGraffle Pro 7.2.2 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
OmniGraffle 7.2.2 - Create diagrams, flo...
OmniGraffle helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use Graffle to... Read more
Spotify 1.0.44.100. - Stream music, crea...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
Microsoft OneNote 15.29 - Free digital n...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
WALTR 2 2.0.8 - $39.95
WALTR 2 helps you wirelessly drag-and-drop any music, ringtones, videos, PDF, and ePub files onto your iPhone, iPad, or iPod without iTunes. It is the second major version of Softorino's critically-... Read more
Dropbox 16.3.27 - Cloud backup and synch...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keep them up-to-date between systems... Read more
EtreCheck 3.1.5 - For troubleshooting yo...
EtreCheck is an app that displays the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to... Read more
Carbon Copy Cloner 4.1.12 - Easy-to-use...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
VueScan 9.5.62 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
SpamSieve 2.9.27 - Robust spam filter fo...
SpamSieve is a robust spam filter for major email clients that uses powerful Bayesian spam filtering. SpamSieve understands what your spam looks like in order to block it all, but also learns what... Read more

Latest Forum Discussions

See All

Track Santa with these three festive app...
Christmas is fast approaching and that means it's time to prepare for Santa's yearly pilgrimage around the globe. Christmas Eve is an exciting time as parents help their kids get ready to welcome Santa. You've got the cookies and milk all planned... | Read more »
Galaxy on Fire 3 and four other fantasti...
Galaxy on Fire 3 - Manticore brings the series back for another round of daring space battles. It's familiar territory for folks who are familiar with the franchise. If you've beaten the game and are looking to broaden your horizons, might we... | Read more »
The best apps for your holiday gift exch...
What's that, you say? You still haven't started your holiday shopping? Don't beat yourself up over it -- a lot of people have been putting it off, too. It's become easier and easier to procrastinate gift shopping thanks to a number of apps that... | Read more »
Toca Hair Salon 3 (Education)
Toca Hair Salon 3 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Winter comes to Darkwood as Seekers Note...
MyTona, based in the chilly Siberian city of Yakutsk, has brought a little festive fun to its hidden object game Seekers Notes: Hidden Mystery. The Christmas update introduces some new inhabitants to players, and with them a chance to win plenty of... | Read more »
Bully: Anniversary Edition (Games)
Bully: Anniversary Edition 1.03.1 Device: iOS Universal Category: Games Price: $6.99, Version: 1.03.1 (iTunes) Description: *** PLEASE NOTE: This game is officially supported on the following devices: iPhone 5 and newer, iPod Touch... | Read more »
PINE GROVE (Games)
PINE GROVE 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: A pine grove where there are no footsteps of people due to continuous missing cases. The case is still unsolved and nothing has... | Read more »
Niantic teases new Pokémon announcement...
After rumors started swirling yesterday, it turns out there is an official Pokémon GO update on its way. We’ll find out what’s in store for us and our growing Pokémon collections tomorrow during the Starbucks event, but Niantic will be revealing... | Read more »
3 reasons why Nicki Minaj: The Empire is...
Nicki Minaj is as business-savvy as she is musically talented and she’s proved that by launching her own game. Designed by Glu, purveyors of other fine celebrity games like cult favorite Kim Kardashian: Hollywood, Nicki Minaj: The Empire launched... | Read more »
Clash of Clans is getting its own animat...
Riding on its unending wave of fame and success, Clash of Clans is getting an animated web series based on its Clash-A-Rama animated shorts.As opposed to the current shorts' 60 second run time, the new and improved Clash-A-Rama will be comprised of... | Read more »

Price Scanner via MacPrices.net

Never Settle for Low Performing Wifi With iOS...
AppYogi Software has announced the release of WiFi Signal Strength Status App 1.0, the company’s new utility developed exclusively for macOS. WiFi Signal Strength Status App features a unique, single... Read more
New 2016 13-inch Touch Bar MacBook Pros in st...
B&H Photo has stock of new 2016 Apple 13″ Touch Bar MacBook Pro models, each including free shipping plus NY sales tax only: - 13″ 2.9GHz/512GB Touch Bar MacBook Pro Space Gray: $1999 - 13″ 2.... Read more
New 2016 15″ Touch Bar MacBook Pros in stock...
B&H Photo has new 2016 Apple 15″ Touch Bar MacBook Pro models in stock today including free shipping plus NY sales tax only: - 15″ 2.7GHz Touch Bar MacBook Pro Space Gray: $2799 - 15″ 2.7GHz... Read more
DietSensor App Targeting Diabetes and Obesity...
DietSensor, Inc., a developer of smart food and nutrition applications designed to fight diabetes and obesity and help improve overall fitness, has announced the launch of its DietSensor app for... Read more
Holiday 2016 13-inch 2.0GHz MacBook Pro sales...
B&H has the non-Touch Bar 13″ MacBook Pros in stock today for $50-$100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz MacBook Pro Space Gray (MLL42LL/A): $1449 $... Read more
Holiday sale: Apple TVs for $51-$40 off MSRP,...
Best Buy has dropped their price on the 64GB Apple TV to $159.99 including free shipping. That’s $40 off MSRP. 32GB Apple TVs are on sale right now for $98 on Sams Club’s online store. That’s $51 off... Read more
12-inch Retina MacBooks, Apple refurbished, n...
Apple has restocked a full line of Certified Refurbished 2016 12″ Retina MacBooks, now available for $200-$260 off MSRP. Refurbished 2015 models are available starting at $929. Apple will include a... Read more
Holiday sale: 12-inch Retina MacBook for $100...
B&H has 12″ Retina MacBooks on sale for $100 off MSRP as part of their Holiday sale. Shipping is free, and B&H charges NY sales tax only: - 12″ 1.1GHz Space Gray Retina MacBook: $1199 $100... Read more
Apple refurbished 13-inch MacBook Airs availa...
Apple has Certified Refurbished 13″ MacBook Airs available starting at $849. An Apple one-year warranty is included with each MacBook, and shipping is free: - 13″ 1.6GHz/8GB/128GB MacBook Air: $849 $... Read more
Apple refurbished iMacs available for up to $...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more

Jobs Board

Integration Technician, *Apple* - Zones, In...
…at Zones and for our customers each day. Position Overview The Apple Integration Technician will be responsible for performing customer specific configuration Read more
*Apple* Brand Ambassador (Macy's) - The...
…(T-ROC), is proud of its unprecedented relationship with our partner and client, APPLE ,in bringing amazing" APPLE ADVOCATES"to "non" Apple store locations. Read more
*Apple* Retail - Multiple Positions- Trumbul...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
US- *Apple* Store Leader Program - Apple (Un...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.