# Multi-line Comet Plot 3

Posted by **Jiro Doke**,

Jiro's pick this week is `multicomet` by Ben.

Did you know that we had a function called `comet` that allows you to animate trajectory plots? It was like a hidden gem for me when I found it. It's a simple concept, but very useful for when I wanted to animate my signal trace. There was no need for me to write loops to update my graphics. With the new toolstrip, you can find it right away.

Ben extended `comet` to work with multiple trajectories. Now it's easy to visualize multi-state dynamics, such as the Lotka-Volterra predator-prey model.

$$y_1' = (1 - \alpha \cdot y_2) \cdot y_1$$

$$y_2' = (-1 + \beta \cdot y_1) \cdot y_2$$

The prey population increases when there are no predators, and the predator population decreases when there are no prey. Let's use the example function `lotka` that uses $\alpha = 0.01$ and $\beta = 0.02$.

```
type lotka
```

function yp = lotka(t,y) %LOTKA Lotka-Volterra predator-prey model. % Copyright 1984-2002 The MathWorks, Inc. % $Revision: 5.7 $ $Date: 2002/04/15 03:33:21 $ yp = diag([1 - .01*y(2), -1 + .02*y(1)])*y;

```
% Define initial conditions.
y0 = [20; 20];
```

```
% Simulate the differential equation.
[t,y] = ode45(@lotka, 0:0.005:15, y0);
```

```
% Animate.
multicomet(t, y)
```

Let's try different initial conditions.

y0 = [200; 200];

[t,y] = ode45(@lotka, 0:0.005:15, y0);

multicomet(t, y)

Thanks, Ben, for this nice extension to `comet`!

**Comments**

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the MATLAB code

Published with MATLAB® R2013a

**Category:**- Picks

## 3 CommentsOldest to Newest

**1**of 3

No problem Jiro! It was a pleasant surprise to see my code had been selected for pick of the week. I’m glad you find it useful!

Ben

**2**of 3

This is nice.

If you continue with

figure

comet(y(:,1),y(:,2))

you will get the phase plane plot. It would be instructive to have simultaneous animations, which would require parallel processing, but how?

**3**of 3

@Heikki,

“comet” by itself won’t let you do what you’re trying to do, but you can have multiple axes/figures do simultaneous animations without parallel processing.

Here‘s an example of something similar to what you are asking. See how the various graphics objects are being updated inside of a loop. The graphics could be on different axes (like in the example) or different figure windows. Note that this example is on creating an animated GIF, so there are some extra code for doing that.

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