matplotlib-users

[Matplotlib-users] Scatter legend with colored marker

[Jorge S. <jor...@ya...>]

Hi,
I am adding several scatter plots to the same axis, each having a specific
color. When I call legend on the axis it correctly picks all scatter plots with
their symbols and labels, but it doesn't pick up the color. The example below
demonstrates this. What I would like to do is to have in the legend the marker
its color matching that of the set it represents. How would I do this? 

Thanks,

Jorge

PS: I am using matplotlib 1.0

-----------
import numpy as np
import matplotlib as mpl
import matplotlib.pyplot as plt

data0 = np.random.rand(10,2)
data1 = np.random.rand(10,2)
data2 = np.random.rand(10,2)
data = [data0, data1, data2]

fig, ax = plt.subplots(1,1)
norm = mpl.colors.Normalize(0,len(data))

for i,d in enumerate(data):
    ax.scatter(d.T[0], d.T[1], label='data set ' + str(i), c=np.ones(d.shape[0])*i,
            norm=norm)
ax.legend()
plt.show()
-----------

Re: [Matplotlib-users] Scatter legend with colored marker

[Jorge S. <jor...@ya...>]

Jorge Scandaliaris <jorgesmbox-ml@...> writes:
> 
> Hi,
> I am adding several scatter plots to the same axis, each having a specific
> color. When I call legend on the axis it correctly picks all scatter plots with
> their symbols and labels, but it doesn't pick up the color. The example below
> demonstrates this. What I would like to do is to have in the legend the marker
> its color matching that of the set it represents. How would I do this? 

Answering my own question, I found a way to achieve what I wanted by using a
proxy artist as suggested in this thread:
http://thread.gmane.org/gmane.comp.python.matplotlib.general/20995/focus=20999

Maybe there's a better approach. I certainly would love legend to pick up on the
color used in the scatter plot by its own.

Jorge

Re: [Matplotlib-users] Scatter legend with colored marker

[Benjamin R. <ben...@ou...>]

On Wed, Aug 4, 2010 at 10:00 AM, Jorge Scandaliaris
<jor...@ya...>wrote:

> Jorge Scandaliaris <jorgesmbox-ml@...> writes:
> >
> > Hi,
> > I am adding several scatter plots to the same axis, each having a
> specific
> > color. When I call legend on the axis it correctly picks all scatter
> plots with
> > their symbols and labels, but it doesn't pick up the color. The example
> below
> > demonstrates this. What I would like to do is to have in the legend the
> marker
> > its color matching that of the set it represents. How would I do this?
>
> Answering my own question, I found a way to achieve what I wanted by using
> a
> proxy artist as suggested in this thread:
>
> http://thread.gmane.org/gmane.comp.python.matplotlib.general/20995/focus=20999
>
> Maybe there's a better approach. I certainly would love legend to pick up
> on the
> color used in the scatter plot by its own.
>
> Jorge
>
>
>
Hmm, this definitely looks like a bug.  If I explicitly state what color I
want using the "color" keyword instead of using 'c' and 'norm', then
everything works properly.

Digging through the code, 'color' gets used right after the creation of the
collection object by calling the .update() function, which in turn calls the
.set_color() function which calls the set_facecolor() and set_edgecolor()
functions.  However, when using 'c' and 'norm', and having 'c' contain an
array that is the same length as the data, causes colors to be set to None
during the creation of the polygon collection.  After the collection
creation, set_array(), set_cmap() and set_norm() are called, which allows
for the scatter graph to be colored correctly upon draw.

However, I suspect that when the legend is being created, the legend uses
whatever the face/edge color that was set for the collection, which probably
isn't set yet and won't be until a draw is performed.

Ben Root

Re: [Matplotlib-users] Scatter legend with colored marker

[Jorge S. <jor...@ya...>]

Benjamin Root <ben.root@...> writes:

<snip>
> Hmm, this definitely looks like a bug.  If I explicitly state what color I want
> using the "color" keyword instead of using 'c' and 'norm', then everything
> works properly.

I can't tell if this is a bug or not, but the trick of using a color directly is
an even simpler workaround than the one I used. Thanks.

Jorge

Re: [Matplotlib-users] Scatter legend with colored marker

[Jorge S. <jor...@ya...>]

Benjamin Root <ben.root@...> writes:

<snip>
> Hmm, this definitely looks like a bug. 

Should I fill a bug report about this?

Jorge

Re: [Matplotlib-users] Scatter legend with colored marker

[Benjamin R. <ben...@ou...>]

On Wed, Aug 4, 2010 at 3:01 PM, Jorge Scandaliaris
<jor...@ya...>wrote:

> Benjamin Root <ben.root@...> writes:
>
> <snip>
> > Hmm, this definitely looks like a bug.
>
> Should I fill a bug report about this?
>
> Jorge
>
>
Yes, please do, and then mention which bug report you filed to this thread.

Ben Root

Re: [Matplotlib-users] Scatter legend with colored marker

[Jorge S. <jor...@ya...>]

Benjamin Root <ben.root@...> writes:

<snip>
> 
> Yes, please do, and then mention which bug report you filed to this thread.
Ben Root

Done. Bug number is 3039678 

Could this behavior be due to the fact that scatter() accepts sequences for
setting the color of each point individually? Maybe I am way off, but who 
knows...

Jorge

Re: [Matplotlib-users] Scatter legend with colored marker

[Benjamin R. <ben...@ou...>]

On Wed, Aug 4, 2010 at 3:52 PM, Jorge Scandaliaris
<jor...@ya...>wrote:

> Benjamin Root <ben.root@...> writes:
>
> <snip>
> >
> > Yes, please do, and then mention which bug report you filed to this
> thread.
> Ben Root
>
> Done. Bug number is 3039678
>
> Could this behavior be due to the fact that scatter() accepts sequences for
> setting the color of each point individually? Maybe I am way off, but who
> knows...
>
> Jorge
>
>
>
Probably not directly, but I hadn't thought about that before.  For a set of
scatter points that are colored by values, what should the legend show?  In
other words, what does it *mean* for there to be a legend for points that
are colored in a potentially non-uniform manner?

So, maybe this is desired behavior (but possibly by accident)?

Thanks for your help,
Ben Root

Re: [Matplotlib-users] Scatter legend with colored marker

[Jorge S. <jor...@ya...>]

Benjamin Root <ben.root@...> writes:
> 
<snip>
> Probably not directly, but I hadn't thought about that before.  For a set of 
> scatter points that are colored by values, what should the legend show?  In 
> other words, what does it *mean* for there to be a legend for points that are 
> colored in a potentially non-uniform manner?So, maybe this is desired behavior 
> (but possibly by accident)?
> Thanks for your help,
> Ben Root

I thought for a moment that legend was using the color of the first point in the
set, but a quick test reveals that no matter what colormap you specify, the
marker color inside the legend is blue. BTW, I think I've found another thing
related to legend() and scatter plots: the 'numpoints' keyword argument to
legend is not respected, as showed in the example pasted below,

Jorge

-------
import numpy as np
import matplotlib as mpl
import matplotlib.pyplot as plt

data0 = np.random.rand(10,2)
data1 = np.random.rand(10,2)
data2 = np.random.rand(10,2)
data = [data0, data1, data2]

fig, ax = plt.subplots(1,1)
norm = mpl.colors.Normalize(0,len(data))
cmap = mpl.cm.afmhot

sc = []
labels = []
for i,d in enumerate(data):
    sc.append(ax.scatter(d.T[0], d.T[1], c=np.ones(d.shape[0])*i,
            norm=norm, cmap=cmap))
    labels.append('data set ' + str(i))
ax.legend(sc, labels, numpoints=1)
plt.show()
-------

Re: [Matplotlib-users] Scatter legend with colored marker

[Benjamin R. <ben...@ou...>]

On Thu, Aug 5, 2010 at 2:48 AM, Jorge Scandaliaris
<jor...@ya...>wrote:

> Benjamin Root <ben.root@...> writes:
> >
> <snip>
> > Probably not directly, but I hadn't thought about that before.  For a set
> of
> > scatter points that are colored by values, what should the legend show?
> In
> > other words, what does it *mean* for there to be a legend for points that
> are
> > colored in a potentially non-uniform manner?So, maybe this is desired
> behavior
> > (but possibly by accident)?
> > Thanks for your help,
> > Ben Root
>
> I thought for a moment that legend was using the color of the first point
> in the
> set, but a quick test reveals that no matter what colormap you specify, the
> marker color inside the legend is blue. BTW, I think I've found another
> thing
> related to legend() and scatter plots: the 'numpoints' keyword argument to
> legend is not respected, as showed in the example pasted below,
>
> Jorge
>
> -------
> import numpy as np
> import matplotlib as mpl
> import matplotlib.pyplot as plt
>
> data0 = np.random.rand(10,2)
> data1 = np.random.rand(10,2)
> data2 = np.random.rand(10,2)
> data = [data0, data1, data2]
>
> fig, ax = plt.subplots(1,1)
> norm = mpl.colors.Normalize(0,len(data))
> cmap = mpl.cm.afmhot
>
> sc = []
> labels = []
> for i,d in enumerate(data):
>     sc.append(ax.scatter(d.T[0], d.T[1], c=np.ones(d.shape[0])*i,
>            norm=norm, cmap=cmap))
>    labels.append('data set ' + str(i))
> ax.legend(sc, labels, numpoints=1)
> plt.show()
> -------
>
>
Yes, this was found a little while back and I believe it was fixed for v1.0.

Ben Root

Re: [Matplotlib-users] Scatter legend with colored marker

[Jae-Joon L. <lee...@gm...>]

On Thu, Aug 5, 2010 at 4:48 PM, Jorge Scandaliaris
<jor...@ya...> wrote:
> BTW, I think I've found another thing
> related to legend() and scatter plots: the 'numpoints' keyword argument to
> legend is not respected, as showed in the example pasted below,
>
> Jorge

In recent versions, there is a separate parameter : scatterpoints. So
check your document.

 ax.legend(sc, labels, scatterpoints=1)

Regards,

-JJ

Re: [Matplotlib-users] Scatter legend with colored marker

[Jorge S. <jor...@ya...>]

Jae-Joon Lee <lee.j.joon@...> writes:

<snip>
> In recent versions, there is a separate parameter : scatterpoints. So
> check your document.
> 
>  ax.legend(sc, labels, scatterpoints=1)
> 
> Regards,
> 
> -JJ

Great! It works fine with 1.0.0

Jorge