Legend using PathCollections in matplotlib

Question

I\'m plotting groups of circles using collections and I am not able to generate the legend of the three categories. I want:

• Cat 1: red circles
• Cat 2: blue

Answer 1

One possible solution is to add Line2D objects to use in the legend, also known as using proxy artists. To achieve this you have 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")