Legend using PathCollections in matplotlib

Question

I draw groups of circles using collections, and I cannot generate a legend about three categories. I want to:

• Cat 1: red circles
• Cat 2: blue circles
• Cat 3: yellow circles

import matplotlib import matplotlib.pyplot as plt from matplotlib.collections import PatchCollection from matplotlib.patches import Circle import numpy as np # (modified from one of the matplotlib gallery examples) resolution = 50 # the number of vertices N = 50 Na = 25 Nb = 10 x = np.random.random(N) y = np.random.random(N) radii = 0.1*np.random.random(30) xa = np.random.random(Na) ya = np.random.random(Na) radiia = 0.1*np.random.random(50) xb = np.random.random(Nb) yb = np.random.random(Nb) radiib = 0.1*np.random.random(60) patches = [] patchesa = [] patchesb = [] for x1,y1,r in zip(x, y, radii): circle = Circle((x1,y1), r) patches.append(circle) for x1,y1,r in zip(xa, ya, radiia): circle = Circle((x1,y1), r) patchesa.append(circle) for x1,y1,r in zip(xb, yb, radiib): circle = Circle((x1,y1), r) patchesb.append(circle) fig = plt.figure() ax = fig.add_subplot(111) colors = 100*np.random.random(N) p = PatchCollection(patches, cmap=matplotlib.cm.jet, alpha=0.4, label= "Cat 1", facecolor="red") pa = PatchCollection(patchesa, cmap=matplotlib.cm.jet, alpha=0.3, label= "Cat 2", facecolor="blue") pb = PatchCollection(patchesb, cmap=matplotlib.cm.jet, alpha=0.4, label= "Cat 3", facecolor="yellow") #p.set_array(colors) ax.add_collection(p) ax.add_collection(pa) ax.add_collection(pb) ax.legend(loc = 2) plt.colorbar(p) print p.get_label() plt.show() 

PathCollection are not PathCollection objects, so they try to generate a legend as follows;

 legend([p, pa, pb], ["cat 1", "2 cat", "cat 3"]) 

does not work.

How can the title appear?

My system runs on Python 2.7 and Matplotlib 1.2.0_1

Note that the print p.get_label() command indicates that the object has an associated label, but matplotlib cannot set the legend.

Answer 1

One possible solution is to add Line2D objects for use in the legend, also known as using proxy artists. To do this, you need to add from matplotlib.lines import Line2D to your script, and then you can replace this code:

 ax.legend(loc = 2) plt.colorbar(p) print p.get_label() 

with this:

 circ1 = Line2D([0], [0], linestyle="none", marker="o", alpha=0.4, markersize=10, markerfacecolor="red") circ2 = Line2D([0], [0], linestyle="none", marker="o", alpha=0.3, markersize=10, markerfacecolor="blue") circ3 = Line2D([0], [0], linestyle="none", marker="o", alpha=0.4, markersize=10, markerfacecolor="yellow") plt.legend((circ1, circ2, circ3), ("Cat 1", "Cat 2", "Cat 3"), numpoints=1, loc="best")