Legend for contour plots

Community matplotlib-users
1

Dear All,

I am using contour plot and I am drawing different contours with
different colors and linestyles and I would like to have a legend with
a caption for each contour function used.
Here you can see an example of what I would like to do

#create the 2D histogram and the x and y axis
x, y = np.random.normal(0, 0.5, 1000), np.random.normal(0, 1, 1000)
h, xe,ye = np.histogram2d(x,y, bins=25)
xe, ye = (xe[1:]+xe[:-1])/2, (ye[1:]+ye[:-1])/2

lines,text = , # initialise lists

#contour plots
lines.append(plt.contour(xe,ye,h, levels=[10,9], linestyles="-", colors="k"))
text.append("level=10, 9")

lines.append(plt.contour(xe,ye,h, levels=[5,4], linestyles="--", colors="r"))
text.append("level=5, 4")

plt.legend(lines, text)

Everything goes well untill I plot the legend. At the end of the mail
I report the error that I get.
Anyway, if I do

plt.legend(lines)

I don't get any errors but it's quite useless, since the text of the
legend is just like:
<matplotlib.contour.ContourSet instance at 0x6bedc20>
as you can see from the attached figure.

I've the feeling that the problem is that "contour" gives back a
"matplotlib.contour.ContourSet instance", while the functions like
"plot" gives back a "<matplotlib.lines.Line2D object".

Does anyone knows how to do what I want?

Thanks in advance

Francesco

ps: Error message follow

example.png
example.png812×612 41.2 KB

···

--------------------------

/usr/lib/pymodules/python2.6/matplotlib/pyplot.pyc in legend(*args, **kwargs)
   2435 # changes will be lost

   2436 def legend(*args, **kwargs):
-> 2437 ret = gca().legend(*args, **kwargs)
   2438 draw_if_interactive()
   2439 return ret

/usr/lib/pymodules/python2.6/matplotlib/axes.pyc in legend(self,
*args, **kwargs)
   4042
   4043 handles = cbook.flatten(handles)
-> 4044 self.legend_ = mlegend.Legend(self, handles, labels, **kwargs)
   4045 return self.legend_
   4046

/usr/lib/pymodules/python2.6/matplotlib/legend.pyc in __init__(self,
parent, handles, labels, loc, numpoints, markerscale, scatterpoints,
scatteryoffsets, prop, pad, labelsep, handlelen, handletextsep,
axespad, borderpad, labelspacing, handlelength, handletextpad,
borderaxespad, columnspacing, ncol, mode, fancybox, shadow, title,
bbox_to_anchor, bbox_transform)
    302
    303 # init with null renderer

--> 304 self._init_legend_box(handles, labels)
    305
    306 self.set_title(title)

/usr/lib/pymodules/python2.6/matplotlib/legend.pyc in
_init_legend_box(self, handles, labels)
    580
    581 handle = handle_list[-1]
--> 582 handlebox.add_artist(handle)
    583 if hasattr(handle, "_legmarker"):
    584 handlebox.add_artist(handle._legmarker)

/usr/lib/pymodules/python2.6/matplotlib/offsetbox.pyc in add_artist(self, a)
    473 'Add any :class:`~matplotlib.artist.Artist` to the
container box'
    474 self._children.append(a)
--> 475 a.set_transform(self.get_transform())
    476
    477

AttributeError: 'NoneType' object has no attribute 'set_transform'

2

Francesco Montesano, on 2011-01-21 15:44, wrote:

Dear All,

I am using contour plot and I am drawing different contours with
different colors and linestyles and I would like to have a legend with
a caption for each contour function used.
Here you can see an example of what I would like to do

>> #create the 2D histogram and the x and y axis
>> x, y = np.random.normal(0, 0.5, 1000), np.random.normal(0, 1, 1000)
>> h, xe,ye = np.histogram2d(x,y, bins=25)
>> xe, ye = (xe[1:]+xe[:-1])/2, (ye[1:]+ye[:-1])/2
>>
>> lines,text = , # initialise lists
>>
>> #contour plots
>> lines.append(plt.contour(xe,ye,h, levels=[10,9], linestyles="-", colors="k"))
>> text.append("level=10, 9")
>>
>> lines.append(plt.contour(xe,ye,h, levels=[5,4], linestyles="--", colors="r"))
>> text.append("level=5, 4")
>>
>> plt.legend(lines, text)

Everything goes well untill I plot the legend. At the end of the mail
I report the error that I get.
Anyway, if I do
>> plt.legend(lines)
I don't get any errors but it's quite useless, since the text of the
legend is just like:
<matplotlib.contour.ContourSet instance at 0x6bedc20>
as you can see from the attached figure.

I've the feeling that the problem is that "contour" gives back a
"matplotlib.contour.ContourSet instance", while the functions like
"plot" gives back a "<matplotlib.lines.Line2D object".

Does anyone knows how to do what I want?

Hi Francesco,

here's one way of getting what you want, instead of calling
legend on your 'lines' variable as you had it, do this:

  actual_lines = [cs.collections[0] for cs in lines]
  plt.legend(actual_lines, text)

As you note, the call to plt.countour does not return lines, it
returns contour sets (which is why I called the variable 'cs' in
my example). Poking around in ipython, I saw that each contour
set has a collections attribute which holds the actual lines.

hope that helps,

···

--
Paul Ivanov
314 address only used for lists, off-list direct email at:
http://pirsquared.org | GPG/PGP key id: 0x0F3E28F7

3

Dear Paul,

Thank you, it does exacly what I want to do. I searched a bit into the
"contour" instance, but I was biased since I was looking for something
like "get_line".

cheers

Francesco

2011/1/21 Paul Ivanov <pivanov314@...287...>:

···

Francesco Montesano, on 2011-01-21 15:44, wrote:

Dear All,

I am using contour plot and I am drawing different contours with
different colors and linestyles and I would like to have a legend with
a caption for each contour function used.
Here you can see an example of what I would like to do

>> #create the 2D histogram and the x and y axis
>> x, y = np.random.normal(0, 0.5, 1000), np.random.normal(0, 1, 1000)
>> h, xe,ye = np.histogram2d(x,y, bins=25)
>> xe, ye = (xe[1:]+xe[:-1])/2, (ye[1:]+ye[:-1])/2
>>
>> lines,text = , # initialise lists
>>
>> #contour plots
>> lines.append(plt.contour(xe,ye,h, levels=[10,9], linestyles="-", colors="k"))
>> text.append("level=10, 9")
>>
>> lines.append(plt.contour(xe,ye,h, levels=[5,4], linestyles="--", colors="r"))
>> text.append("level=5, 4")
>>
>> plt.legend(lines, text)

Everything goes well untill I plot the legend. At the end of the mail
I report the error that I get.
Anyway, if I do
>> plt.legend(lines)
I don't get any errors but it's quite useless, since the text of the
legend is just like:
<matplotlib.contour.ContourSet instance at 0x6bedc20>
as you can see from the attached figure.

I've the feeling that the problem is that "contour" gives back a
"matplotlib.contour.ContourSet instance", while the functions like
"plot" gives back a "<matplotlib.lines.Line2D object".

Does anyone knows how to do what I want?

Hi Francesco,

here's one way of getting what you want, instead of calling
legend on your 'lines' variable as you had it, do this:

actual_lines = [cs.collections[0] for cs in lines]
plt.legend(actual_lines, text)

As you note, the call to plt.countour does not return lines, it
returns contour sets (which is why I called the variable 'cs' in
my example). Poking around in ipython, I saw that each contour
set has a collections attribute which holds the actual lines.

hope that helps,
--
Paul Ivanov
314 address only used for lists, off-list direct email at:
http://pirsquared.org | GPG/PGP key id: 0x0F3E28F7

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