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

Tips and tricks to get a higher score in...
Tank.iois - you guessed it! - another multiplayer arena battler likeAgar.io and Slither.io. It does differentiate itself by putting you in a tiny tank though, so it's not exactly the same. To help you get that all-important high score, we've got a... | Read more »
How to unlock characters in One Tap Tenn...
As the title suggests, One Tap Tennis requires only a single tap to play its particular brand of tennis, and rewards you with a ton of unlockable characters if you perform well. Fortunately for you, we at 148Apps have got a few tips and tricks to... | Read more »
Grab it now: Game Craft’s Legend of War...
The real time strategy game is now available for you to sink your teeth into, through the App Store and Google Play. Combining elements of skill, strategy and empire building, Legend of War is a real gamers’ game. [Read more] | Read more »
Skateboard Party 3 ft. Greg Lutzka (Gam...
Skateboard Party 3 ft. Greg Lutzka 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Skateboard Party is back! This third edition of the popular sports franchise features professional skater... | Read more »
Cubious (Games)
Cubious 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Cubious – How smart are you? How high is your IQube? Solve the impossible puzzles to find out, and help a lost little cube find his... | Read more »
Goat Simulator Waste of Space (Games)
Goat Simulator Waste of Space 1.1 Device: iOS Universal Category: Games Price: $4.99, Version: 1.1 (iTunes) Description: ** IMPORTANT - SUPPORTED DEVICESiPhone 4S, iPad 2, iPod Touch 5 or better.** | Read more »
Wildfulness - Unwind in nature and calm...
Wildfulness - Unwind in nature and calm your mind with nature sounds and illustrations 1.0 Device: iOS Universal Category: Healthcare & Fitness Price: $1.99, Version: 1.0 (iTunes) Description: Spending time in nature helps you to... | Read more »
Dr. Panda Racers (Education)
Dr. Panda Racers 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: STEP ON THE GAS, RACE AND WIN!Fasten your seat belts and get ready to race! Speed your way to the finish line while doing... | Read more »
ROMANCING SAGA 2 (Games)
ROMANCING SAGA 2 1.0.0 Device: iOS Universal Category: Games Price: $17.99, Version: 1.0.0 (iTunes) Description: Romancing SaGa 2, originally released only in Japan in 1993, has been completely remastered and now receives its first... | Read more »
WRIO Keyboard (Utilities)
WRIO Keyboard 1.0 Device: iOS iPhone Category: Utilities Price: $2.99, Version: 1.0 (iTunes) Description: 40% OFF DURING LIMITED INTRODUCTORY OFFER | Read more »

Price Scanner via MacPrices.net

Enterprise Workers Pick Technology Over Perks...
New Adobe study shows surprising attitudes about office jobs and where the future of work is heading. Adobe has released survey findings revealing that a surprising 70 percent of U.S. office workers... Read more
Goal Zero and OtterBox Partner to Expand iPh...
Goal Zero, specialists in portable power, have announced a partnership with OtterBox, brand smartphone case protection, to offer the Slide and Slide Plus Batteries as modules compatible with the new... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $210 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Clearance 2015 13-inch MacBook Airs available...
B&H Photo has clearance 2015 13″ MacBook Airs available for $250 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/4GB/128GB MacBook Air (MJVE2LL/A): $799... Read more
Apple refurbished Apple TVs available for up...
Apple has Certified Refurbished 32GB and 64GB Apple TVs available for up to $30 off the cost of new models. Apple’s standard one-year warranty is included with each model, and shipping is free: -... Read more
21-inch iMacs on sale for up to $120 off MSRP
B&H Photo has 21″ iMacs on sale for up to $120 off MSRP including free shipping plus NY sales tax only: - 21″ 3.1GHz iMac 4K: $1379.99 $120 off MSRP - 21″ 2.8GHz iMac: $1189 $110 off MSRP - 21″ 1... Read more
Kanex Introduces GoPower USB-C Rechargeable B...
Kanex has announced its GoPower USB-C portable battery for the USB-C MacBook, featuring the new industry standard connector and cable used for connectivity and power. Providing users with a new... Read more
Convertible and Detachable Devices Winning Ov...
According to the latest figures published by International Data Corporation (IDC), Western European shipments of ultraslim convertibles and detachables posted positive growth (44.7%) to account for... Read more
New MacBook Pros And Will MacBook Air Be Upgr...
With my mid-2013 13-inch MacBook Air closing on its third anniversary come November, I’m in system upgrade mode. Actually the Haswell CPU equipped Air is still doing a fine job, but my good wife is... Read more
Apple’s Education discount saves up to $300 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more

Jobs Board

Service Assistant - *Apple* Chevrolet *App...
Apple Automotive is one of the fastest growing dealer...and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive, we Read more
Editor, *Apple* News - APPLE (United States...
Job Summary The Apple News team is looking for a passionate and knowledgeable editor with experience covering entertainment/pop culture and experience running social Read more
*Apple* Nissan Service Technicians - Apple A...
Apple Automotive is one of the fastest growing dealer...and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive , Read more
ISCS *Apple* ID Site Support Engineer - APP...
…position, we are looking for an individual who has experience supporting customers with Apple ID issues and enjoys this area of support. This person should be Read more
Automotive Sales Consultant - Apple Ford Linc...
…you. The best candidates are smart, technologically savvy and are customer focused. Apple Ford Lincoln Apple Valley is different, because: $30,000 annual salary Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.