Plotting Data

One of the most powerful features in GnuPlot is how easily Data can be plotted. For example, we have the following data file (in /opt/Gnu/info/gp/example1.dat). The data was generated by applying euler's method to a ODE problem. The first column is the number of subintervals. The second column is the step size (1/(# of subintervals)). The third column is the value obtained, and the fourth column is the absolute error.

# Example1.dat
# number of subint.   - width of subinterval, computed value, abs. error
0 1			5		 0.00673794699908559
1 0.5			5.0009765625	 0.00576138449908559
2 0.25			5.00317121193893 0.00356673506015159
3 0.125			5.00478985229103 0.00194809470805701
4 0.0625		5.00572403277733 0.0010139142217529
5 0.03125		5.00622120456923 0.000516742429855555
6 0.015625		5.00647715291715 0.000260794081935245
7 0.0078125		5.00660694721608 0.000130999783004349
8 0.00390625		5.00667229679632 6.56502027673866e-05
9 0.001953125		5.0067050843668  3.28626322820824e-05
10 0.0009765625		5.0067215063061  1.64406929883398e-05
11 0.00048828125	5.00672972430911 8.22268997247022e-06
12 0.000244140625	5.00673383506831 4.11193077187733e-06
13 0.0001220703125	5.00673589088727 2.05611181058885e-06
14 6.103515625e-05	5.00673691890656 1.02809252577885e-06
15 3.0517578125e-05	5.00673743294363 5.14055459532869e-07
16 1.52587890625e-05	5.00673768996904 2.57030048800289e-07
17 7.62939453125e-06    5.00673781848355 1.28515534214557e-07
18 3.814697265625e-06   5.00673788274127 6.42578132925564e-08
19 1.9073486328125e-06  5.00673791487013 3.21289537197345e-08
20 9.5367431640625e-07  5.00673793093482 1.60642699142954e-08
21 4.76837158203125e-07 5.00673793896686 8.03222910406021e-09

Note that lines beginning with a # are considered comment lines and are ignored. To plot this data, you can type the following: (if the file isn't in the first level of your home directory, you can specify the full path, or see the directions on Saving and Loading From a file for information on changing directories.)

plot "example1.dat"

This plots points (x, y) where x is from the first column and y is from the second column. To make the graph a little easier to read, you could type the following:(for information on these, see logscale, Line Style)

set logscale
set data style linespoints
replot

Changing which columns are plotted

GnuPlot also lets you plot data from differant columns against each other. You add the using keywork to a plot command. For example, to plot the computed value (column 3) against the number of subintervals (column 1), you could type:

set nologscale
plot "example1.dat" using 1:3

Or to plot the third column against the 4th column,

plot "exampl1.dat" using 4:3

There are many advanced options for plotting columns against each other like this, for more information see help plot datafile using.

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College of Natural Sciences / University of Northern Iowa / [email protected]