How do you create a legend for a contour plot?

contour, legend, matplotlib, python

Solution

You can create proxy artists to make the legend:

import numpy as np
import matplotlib.pyplot as plt
x, y = np.meshgrid(np.arange(10),np.arange(10))
z = np.sqrt(x**2 + y**2)
cs = plt.contourf(x,y,z,levels=[2,3,4,6])

proxy = [plt.Rectangle((0,0),1,1,fc = pc.get_facecolor()[0]) 
    for pc in cs.collections]

plt.legend(proxy, ["range(2-3)", "range(3-4)", "range(4-6)"])
plt.show()

Problem

I can't seem to find the answer anywhere! I found a discussion here, but trying this I get a `TypeError: 'NoneType' object is not iterable`: ``` >>> import numpy as np >>> import matplotlib.pyplot as plt >>> x, y = np.meshgrid(np.arange(10),np.arange(10)) >>> z = x + y >>> cs = plt.contourf(x,y,z,levels=[2,3]) >>> cs.collections[0].set_label('test') >>> plt.legend() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/pyplot.py", line 2791, in legend ret = gca().legend(*args, **kwargs) File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/axes.py", line 4475, in legend self.legend_ = mlegend.Legend(self, handles, labels, **kwargs) File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/legend.py", line 365, in __init__ self._init_legend_box(handles, labels) File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/legend.py", line 627, in _init_legend_box handlebox) File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/legend_handler.py", line 110, in __call__ handlebox.get_transform()) File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/legend_handler.py", line 352, in create_artists width, height, fontsize) File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/legend_handler.py", line 307, in get_sizes size_max = max(orig_handle.get_sizes())*legend.markerscale**2 TypeError: 'NoneType' object is not iterable ``` EDIT: I'm looking for something like this:

Original source