#### Overview of this book

gnuplot is the world's finest technical plotting software, used by scientists, engineers, and others for many years. It is in constant development and runs on practically every operating system, and can produce output in almost any format. The quality of its 3d plots is unmatched and its ability to be incorporated into computer programs and document preparation systems is excellent. gnuplot Cookbook ñ it will help you master gnuplot. Start using gnuplot immediately to solve your problems in data analysis and presentation. Quickly find a visual example of the graph you want to make and see a complete, working script for producing it. Learn how to use the new features in gnuplot 4.4. Find clearly explained, working examples of using gnuplot with LaTeX and with your own computer programming language. You will master all the ins and outs of gnuplot through gnuplot Cookbook. You will learn to plot basic 2d to complex 3d plots, annotate from simple labels to equations, integrate from simple scripts to full documents and computer progams. You will be taught to annotate graphs with equations and symbols that match the style of the rest of your text, thus creating a seamless, professional document. You will be guided to create a web page with an interactive graph, and add graphical output to your simulation or numerical analysis program. Start using all of gnuplot's simple to complex features to suit your needs, without studying its 200 page manual through this Cookbook.
gnuplot Cookbook
Credits
www.PacktPub.com
Preface
Free Chapter
Plotting Curves, Boxes, Points, and more
Annotating with Labels and Legends
Applying Colors and Styles
Combining Multiple Plots
Programming gnuplot and Dealing with Data
The Third Dimension
Using and Making Graphical User Interfaces
Surveying Special Topics
Finding Help and Information
Index

## Making a vector plot

When the quantity that depends on x and y has both magnitude and a direction it can be represented by an arrow whose length is proportional to the magnitude. The x-y plane can be broken into a regular grid and an arrow drawn in the plane, representing the direction and magnitude associated with each point on the grid—a vector plot. As we are associating two quantities, the magnitude and direction, or, equivalently, Δx and Δy, for each x-y pair, we can think of this type of plot as a 4D plot.

The previous figure is a vector field of a rotating flow. The exact appearance of the arrows will vary with the output terminal.

### How to do it…

The following script draws the vector plot in the previous figure:

```set xrange [0:pi]
set yrange [0:pi]
set iso 20
set samp 20
unset key
a = .2
plot '++' using 1:2:(-a*sin(\$1)*cos(\$2)):(a*cos(\$1)*sin(\$2))\
w vec size  .06, 15 filled
```

### How it works…

Vector plots require a data file or equivalent pseudofile. Four columns are used for x, y...