# GNUplot Tutorial

http://www.gnuplot.info/

https://sourceforge.net/projects/gnuplot/

When installing gnuplot, be careful to de-select any useless items like Yahoo! toolbars or other ad-ware. You will also need a basic text editor for creating and editing data files: Notepad (Windows), TextEdit (Mac), or emacs (Linux). You probably already have this on your computer.

Install gnuplot on your machine, then work through the following exercises to gain a basic working knowledge of the environment.

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## Plotting Functions

Open gnuplot. Do not use the console version; you should see a screen like this. At the prompt, enter the following commands and see what happens:

plot sin(x)

splot sin(x)              Using the mouse, grab the plot and move it around.

plot sin(x), cos(x)

splot sin(x) + cos(x)

plot exp(-x)

With the up-arrows on your keyboard, you can recall previous entries and edit them.

You can also define variables and functions. Note that gnuplot uses ** to raise a number to a power.

a=2

b=4

c= -8

f(x)= a*x**2 + b*x + c

plot f(x)

c=-300

plot [x=-6:10] f(x)

set yrange [-400:-100]

replot

Of course, plots need labels

set xlabel ‘x’

set ylabel ‘y’

set title ‘example plot’

replot

Common functions are listed below

Function   Returns

abs(x)   absolute value of x, |x|

exp(x)   exponential function of x, base e

sqrt(x)  the square root of x

cos(x)   cosine of x, x is in radians

sin(x)  sine of x, x is in radians

tan(x)   tangent of x, x is in radians

log(x)   log of x, base e

log10(x)  log of x, base 10

## Working With Files

You can save your work to a file load it again the next time you run gnuplot. First, in the gnuplot window, select File / show Current Directory. This indicates which directory gnuplot is using. You may change it as necessary.

In the gnuplot window, select File/Save, then give your work a name: filename.plt

You have to enter “.plt” extension yourself. You can now close and then restart gnuplot. Then, from the File menu, you can open your saved work.

Data can be imported from a text file. You can create a text file using a text editor: Notepad (PC) or TextEdit (Mac). Open your text editor, then cut and past the following into it.

#time (sec) distance (m) uncertainty (m)
0     0.00     0.001
1     0.25     0.1
2     0.40     0.05
3     0.75     0.4
4     1.25     0.2
5     1.30     0.3
6     1.55     0.33
7     2.05     0.1
8     2.20     0.6

The # indicates a comment line. Any line starting with # will be ignored by gnuplot. Save this file to your current directory as example.txt. Then enter the following in gnuplot.

plot ‘example.txt’

Your previous range settings may still be active. From the plotting toolbar . . . select the autoscale tool, then replot.

To include the error bars . . .

plot ‘example.txt’ with yerrorbars

Let’s consider a more complicated data file. Copy the following to a text file ‘example2.txt’.

#x     y1     y2     errorx     errory1     errory2
0     0.00     1.10     1     0.01    0.03
1     0.25     1.20     1     0.1      0.2
2     0.40     1.50     1     0.05    0.05
3     0.75     1.65     1     0.4      0.3
4     1.25     2.30     1     0.2      0.1
5     1.30     2.30     1     0.3      0.4
6     1.55     2.65     1     0.3      0.2
7     2.05     2.90     1     0.1      0.2
8     2.20     3.20     1     0.6      0.3

Enter the following commands

plot ‘example2.txt’ using 1:2:4:5 with xyerrorbars

plot ‘example2.txt’ using 1:2:4:5 with xyerrorbars, ‘example2.txt’ using 1:3:4:6 with xyerrorbars

## Fitting Data to a Function

Let’s fit the data in example.txt to a line. Define the following function

m=1

b=1

g(x) = m*x + b

plot g(x), ‘example.txt’

Now, we will use gnuplot to adjust the variables m and b to fit the data inexample.txt.

fit g(x) ‘example.txt’ via m,b

replot

Consider example2.txt. Here, we must tell gnuplot exactly what data to fit.

fit g(x) ‘example2.txt’ using 1:3 via m,b

plot ‘example2.txt’ using 1:2:4:5 with xyerrorbars, ‘example2.txt’ using 1:3:4:6 with xyerrorbars, g(x)

## Further Help

This tutorial is just enough to get you started. Much more help is available online.