 # drawing circles with scatter

Hello,

I'm trying to draw circles with the scatter function. They are supposed to represent trees in the savannah. It is thus important that they are displayed with a proper size, that is, one which represents their actual size on the field. After quite some confusion, I've found out (I think) that the size argument one can specify with the scatter function is given as a disk's surface in pixels square (I think that's what means the "points^2" in the documentation and from my own tests)

What I would like is to give a surface in unit^2, where "unit" is the unit of my data, and which you can read on the plot's axes ticks. For example, each tree has coordinates like x=3500, y=2210. (The unit here is centimeters but we don't really need to know this). Say I want to draw a tree which canopy is 200 cm wide. That makes a disk which radius is 100, or surface 100^2*PI. How can I draw this?

Many thanks,
Guillaume

2010/9/7 Guillaume Chérel <guillaume.c.cherel@…1972…>

Hello,

I’m trying to draw circles with the scatter function. They are supposed

to represent trees in the savannah. It is thus important that they are

displayed with a proper size, that is, one which represents their actual

size on the field. After quite some confusion, I’ve found out (I think)

that the size argument one can specify with the scatter function is

given as a disk’s surface in pixels square (I think that’s what means

the “points^2” in the documentation and from my own tests)

What I would like is to give a surface in unit^2, where “unit” is the

unit of my data, and which you can read on the plot’s axes ticks. For

example, each tree has coordinates like x=3500, y=2210. (The unit here

is centimeters but we don’t really need to know this). Say I want to

draw a tree which canopy is 200 cm wide. That makes a disk which radius

is 100, or surface 100^2*PI. How can I draw this?

Many thanks,

Guillaume

Guillaume,

Using scatter is probably not the way to go about what you want. The circles for scatter are a fixed size and if you zoom in, they will not scale accordingly.

You probably want to create patches of Circles:

Or even utilize a collection of Circles:

http://matplotlib.sourceforge.net/api/collections_api.html#matplotlib.collections.CircleCollection

Note that for a CircleCollection, you would use ‘offset’ to indicate the center of each circle. After creating the collection, you would then use ax.add_collection() function to add that collection to the axes.

Ben Root

It works great with patches of circles. Thank you.

``````Also, I want my circles to look round, so I use the command
``````

axis(‘equal’). Is there any way to make sure that the area I defined
with xlim() and ylim() won’t be cut off. I’d rather have one
dimension expanded than the other one shrunk. Can I control that?

``````thanks,

guillaume
``````
···

Le 07/09/2010 18:05, Benjamin Root a écrit :

2010/9/7 Guillaume Chérel <guillaume.c.cherel@…287…>

Hello,

``````      I'm trying to draw circles with the scatter function. They are
``````

supposed

``````      to represent trees in the savannah. It is thus important that
``````

they are

``````      displayed with a proper size, that is, one which represents
``````

their actual

``````      size on the field. After quite some confusion, I've found out
``````

(I think)

``````      that the size argument one can specify with the scatter
``````

function is

``````      given as a disk's surface in pixels square (I think that's
``````

what means

``````      the "points^2" in the documentation and from my own tests)

What I would like is to give a surface in unit^2, where "unit"
``````

is the

``````      unit of my data, and which you can read on the plot's axes
``````

ticks. For

``````      example, each tree has coordinates like x=3500, y=2210. (The
``````

unit here

``````      is centimeters but we don't really need to know this). Say I
``````

want to

``````      draw a tree which canopy is 200 cm wide. That makes a disk
``````

``````      is 100, or surface 100^2*PI. How can I draw this?

Many thanks,

Guillaume
``````
``````      Guillaume,

Using scatter is probably not the way to go about what you
``````

want. The circles for scatter are a fixed size and if you
zoom in, they will not scale accordingly.

``````      You probably want to create patches of Circles:

[http://matplotlib.sourceforge.net/api/artist_api.html#matplotlib.patches.Circle](http://matplotlib.sourceforge.net/api/artist_api.html#matplotlib.patches.Circle)

[http://www.mail-archive.com/matplotlib-users@lists.sourceforge.net/msg06786.html](http://www.mail-archive.com/matplotlib-users@lists.sourceforge.net/msg06786.html)

Or even utilize a collection of Circles:

[http://matplotlib.sourceforge.net/api/collections_api.html#matplotlib.collections.CircleCollection](http://matplotlib.sourceforge.net/api/collections_api.html#matplotlib.collections.CircleCollection)

Note that for a CircleCollection, you would use 'offset' to
``````

indicate the center of each circle. After creating the
collection, you would then use ax.add_collection() function to
add that collection to the axes.

``````      I hope that is helpful.

Ben Root
``````

It works great with patches of circles. Thank you.

``````Also, I want my circles to look round, so I use the command
``````

axis(‘equal’). Is there any way to make sure that the area I defined
with xlim() and ylim() won’t be cut off. I’d rather have one
dimension expanded than the other one shrunk. Can I control that?

``````thanks,

guillaume
``````
···

Le 07/09/2010 18:05, Benjamin Root a écrit :

2010/9/7 Guillaume Chérel <guillaume.c.cherel@…287…>

Hello,

``````      I'm trying to draw circles with the scatter function. They are
``````

supposed

``````      to represent trees in the savannah. It is thus important that
``````

they are

``````      displayed with a proper size, that is, one which represents
``````

their actual

``````      size on the field. After quite some confusion, I've found out
``````

(I think)

``````      that the size argument one can specify with the scatter
``````

function is

``````      given as a disk's surface in pixels square (I think that's
``````

what means

``````      the "points^2" in the documentation and from my own tests)

What I would like is to give a surface in unit^2, where "unit"
``````

is the

``````      unit of my data, and which you can read on the plot's axes
``````

ticks. For

``````      example, each tree has coordinates like x=3500, y=2210. (The
``````

unit here

``````      is centimeters but we don't really need to know this). Say I
``````

want to

``````      draw a tree which canopy is 200 cm wide. That makes a disk
``````

``````      is 100, or surface 100^2*PI. How can I draw this?

Many thanks,

Guillaume
``````
``````      Guillaume,

Using scatter is probably not the way to go about what you
``````

want. The circles for scatter are a fixed size and if you
zoom in, they will not scale accordingly.

``````      You probably want to create patches of Circles:

[http://matplotlib.sourceforge.net/api/artist_api.html#matplotlib.patches.Circle](http://matplotlib.sourceforge.net/api/artist_api.html#matplotlib.patches.Circle)

[http://www.mail-archive.com/matplotlib-users@lists.sourceforge.net/msg06786.html](http://www.mail-archive.com/matplotlib-users@lists.sourceforge.net/msg06786.html)

Or even utilize a collection of Circles:

[http://matplotlib.sourceforge.net/api/collections_api.html#matplotlib.collections.CircleCollection](http://matplotlib.sourceforge.net/api/collections_api.html#matplotlib.collections.CircleCollection)

Note that for a CircleCollection, you would use 'offset' to
``````

indicate the center of each circle. After creating the
collection, you would then use ax.add_collection() function to
add that collection to the axes.

``````      I hope that is helpful.

Ben Root
``````

2010/9/8 Guillaume Chérel <guillaume.c.cherel@...287...>:

It works great with patches of circles. Thank you.

Also, I want my circles to look round, so I use the command axis('equal').
Is there any way to make sure that the area I defined with xlim() and ylim()
won't be cut off. I'd rather have one dimension expanded than the other one
shrunk. Can I control that?

You can make it so that axes box itself is changed instead of your data limits:

import matplotlib.pyplot as plt
ax = plt.gca()
ax.set_aspect('equal','box')

Ryan

···

--
Ryan May