PHYS 124 - Freshman Project

Phys 124 - Spring
Gnuplot information

Suppose you have some measurements which you have put in a file data1 which looks like this:

#x  y     error 
 0  0     0 
 1  0.11  0.01
 2  0.39  0.02 
 3  0.92  0.02 
 4  1.59  0.03

The line which starts with a # is a comment line.

To generate a plot of second column with respect to the first one just enter
plot "data1"

If the third column is to be interpreted as possible error, enter
plot "data1" with errorbars
Some of the parts of the command may be shortened:
p "data1" w errorbars
is equivalent.

To plot, for example third column as a function of the first column,
p "data1" using 1:3
or
p "data1" u 1:3
is equivalent.

You can also use using to form functions of the data values for plots. For example,
p "data1" u ($1*$1):2
will plot the second column as a function of the square of the first column.

To plot with lines instead of points,
p "data1" with lines
or
p "data1" w l
is equivalent.

To change the ranges of the axes,
p [1:5][-1:] "data1" with linepoints
or
p [1:5][-1:] "data1" w lp
is equivalent.

To superimpose a second plot, just add another set of parameters after a comma:
p "data1" w lp , 0.1*x*x

You can add labels to x and y axes, to the different curves, etc:
set label "j" at 1,0.6
set xlabel "time"
set ylabel "displacement"
p "data1" w lp title "second column"
When you have to enter this many commands to generate a plot, it becomes more practical to put these commands into a file, and then load them in gnuplot. For example, create a file named Figure1.gp with the following contents:
set label "j" at 1,0.6 set xlabel "time" set ylabel "displacement" p "data1" w lp title "second column"
and then just enter
load "Figure1.gp"
in gnuplot. This way, you will not have to re-write all commands when you want to make adjustments to the figure.

Once you have given your figure its final shape, you can add two lines to your Figure1.gp file as shown below to generate the plot in an image file:

# You can use comment lines that start with a #
set term jpeg
set out "Fig1.jpg"

set label "j" at 1,0.6
set xlabel "time"
set ylabel "displacement"
p "data1" w lp title "second column"

This will result with the figure in the file Fig1.jpg (instead of on the screen) when you enter

l "Figure1.gp"

in gnuplot.

To generate a plot of second column with respect to the first one just enter plot "data1"
If the third column is to be interpreted as possible error, enter plot "data1" with errorbars Some of the parts of the command may be shortened: p "data1" w errorbars is equivalent.
To plot, for example third column as a function of the first column, p "data1" using 1:3 or p "data1" u 1:3 is equivalent.
You can also use using to form functions of the data values for plots. For example, *p "data1" u ($1$1):2** will plot the second column as a function of the square of the first column.
To plot with lines instead of points, p "data1" with lines or p "data1" w l is equivalent.
To change the ranges of the axes, p [1:5][-1:] "data1" with linepoints or p [1:5][-1:] "data1" w lp is equivalent.
To superimpose a second plot, just add another set of parameters after a comma: *p "data1" w lp , 0.1xx*
You can add labels to x and y axes, to the different curves, etc: set label "j" at 1,0.6 set xlabel "time" set ylabel "displacement" p "data1" w lp title "second column" When you have to enter this many commands to generate a plot, it becomes more practical to put these commands into a file, and then load them in gnuplot. For example, create a file named Figure1.gp with the following contents: set label "j" at 1,0.6 set xlabel "time" set ylabel "displacement" p "data1" w lp title "second column" and then just enter load "Figure1.gp" in gnuplot. This way, you will not have to re-write all commands when you want to make adjustments to the figure.