# Gnuplotting

Create scientific plots using gnuplot

October 17th, 2012 | 5 Comments

Some data could be nicely visualized by representing them as arrows. For example, assume that we have done an experiment where we played something to a subject through three loudspeakers and the subject should name the direction where he or she perceived it. In Fig. 1 we show the named direction by the direction of the arrows. The color of the arrow indicates the deviation from the desired direction. A white and not visible arrow means no deviation and a dark red one a deviation of 40° or more.

In gnuplot the `with vectors` command enables the arrows in the plot. It requires four parameters, x, y, dx, dy, where dx and dy controls the endpoint of the arrow as offset values to x,y. In our example the direction is stored as an angle, hence the following functions do the conversion to dx,dy. 0.1 defines the length of the arrows.

```xf(phi) = 0.1*cos(phi/180.0*pi+pi/2)
yf(phi) = 0.1*sin(phi/180.0*pi+pi/2)
```

An optional fifth parameter controls the color of the vector together with the `lc palette` setting. The arrows start at x-dx,y-dy and point to x+dx,y+dy.

```plot 'localization_data.txt' \
u (\$1-xf(\$3)):(\$2-yf(\$3)):(2*xf(\$3)):(2*yf(\$3)):4 \
with vectors head size 0.1,20,60 filled lc palette
```

1. Doug says:

What structure is contained in the localization_data.txt?

2. Doug says:

Nevermind, I see the example file now. Thanks!

3. Doug says:

Thanks for the great example. I’m curious if gnuplot can also show a scale for arrow fill palette.

4. hagen says:

Hi Doug.

I do not exactly know what you mean with “scale for arrow foll palette”, but you can try to change the line

```unset colorbox
```

to

```set colorbox
```

This will add a colorbox to your figure (have a look at this example) showing the scale used for the color of the arrows.

5. Nira says:

Hi All, This example is a so valuable one. I want to make a box around the graph. And is there anyway to change the color of the arrow based on the magnitude of vectors. Can anyone help for me please. thanks. Nira.