Part 1 : Introduction

Motivation

• Gnuplot is a program for creating plots of data.
• It has a command line interface, which means it can be scripted.
• Gnuplot is both simple and powerful.
  • Simple enough to plot data quickly.
  • Powerful enough to create publication quality graphs.

Can’t I just use Excel?

Yes, yes you can.

But…

This animated gif was created using gnuplot and the convert command

we can script Gnuplot!!

Getting Started

• If gnuplot is not already installed, install it.

$ apt search gnuplot
$ apt search gnuplot-x11
...
$ apt install gnuplot-x11
[sudo] password for rshackleford:

Plotting Data

• Go into the sandbox directory.
• Start gnuplot with the gnuplot command.
• Now you are at the gnuplot prompt. Gnuplot is interpreting commands, not the shell.
• The plot command will plot data in a file

gnuplot> plot 'example.txt'

• The set command is used to configure your plot
  • set xlabel "text"
  • set ylabel "text"
  • set title "text"

Demo : Introduction

./demos/01-intro.sh

Part 2 : Plotting Data

plot Command

The plot command takes 1 or more arguments, which take one or more options

gnuplot> plot 'example.txt', 'multi-column.txt'

options:

• title : set the text that will appear in the key
• linewidth : set the line width used
• using : set the columns that will be plotted
  • Data files can contain multiple columns.
  • By default, gnuplot plots the second column, vs the first column (y vs. x)
  • You can select the column to use for x and y with the using option.

  gnuplot> plot 'multi-column.txt' using 1:3

• with : plot using lines, points, or both (linespoints)

Demo : Plot Options

./demos/02-plot_options.sh

Part 3 : Plotting Functions

plotting functions

• Gnuplot can also plot function.
• Just write a calculation that uses ‘x’ for the independent variable.

Functions and Operations:

• \(2x\) -> “2*x”
• \(\sin(x)\) -> “sin(x)”
• \(e^x\) -> “exp(x)”
• \(x^2\) -> “x**2”
• \(\frac{x + 2}{x - 2}\) -> “(x + 2)/(x - 2)”

plotting functions

You can also define your own functions:

gnuplot> f(x) = 10*x + 2
gnuplot> plot f(x)
gnuplot> g(t) = 2*t - 9.8*t**2 / 2
gnuplot> plot g(x)

Demo : plotting functions

./demos/03-plotting_functions.sh

Part 4 : Scripts

scripts

Gnuplot reads command from the command line. It can also read commands from a file.

gnuplot> load "plot-data.txt"

Demo : scripts

./demos/04-scripts.sh

Part 5 : Saving Images

Creating images

• Gnuplot can output graphs to many different formats, which it calls “terminals”
• There are several terminal for outputting graphs to image files
  • png - create PNG image file.
  • jpeg - create JPEG image file.
  • postscript - create postscript image file.
  • x11 - create graph in a window
  • dumb - create an ASCII image of the graph in the terminal.

Demo : Creating images

./demos/05-images.sh

Plot settings

• There are many (dozens at least) of settings that affect how a graph is displayed.
• Most of these settings are set with the set command.
  • gnuplot> set xlabel "time (s)"
  • gnuplot> set grid
  • gnuplot> set xrange [0:1]
  • gnuplot> set xtics 0.1
• Gnuplot has a built-in help. You can read this to see what a certain setting does.
  • gnuplot> help set grid

Demo : Plot settings

./demos/06-settings.sh

Transforming Columns

• The using option can also transform column values before plotting them.

gnuplot> plot 'data.txt' using 1:($2*$2)
gnuplot> plot 'data.txt' using ($1-10):($1*$2)

• For example, recall that the power dissipated by a resistors is

\(P = i^2 R\)

Demo : Column Transformations

./demos/07-column_transform.sh

Part 6: Variables

Variables

• variables can be used to store numerical values.

gnuplot> m = 2
gnuplot> print m
2
gnuplot> plot m*x

Part 8: Error Bars

Error bars

• Error bars can be added to data points in the \(x\) direction, \(y\) direction, or both.

gnuplot> plot 'data.txt' with yerrorbars
gnuplot> plot 'data.txt' with xerrorbars
gnuplot> plot 'data.txt' with xyerrorbars

• We can use column transformations to help…

Demo : Error bars

./demos/11-error_bars.sh

Example 1: Torque and Horsepower

• Car engines generate torque, and produce horsepower.
• Recall that
  • \(P = \frac{dE}{dt}\)
  • \(E = F d = \frac{F}{r} r \theta = \tau \theta\)
  • \(P = \frac{d}{dt} \tau \theta = \tau \omega\)
• So, the power produced by an engine is equal to the torque generated, multiplied by the angular velocity.