Label plotted ellipses

Question

I'm sure this is a basic question, but I can't find the solution. I'm plotting some ellipses and would like to add a legend (something like color of first ellipse: data 1,...) At the moment I manage to plot some ellipses, but I don't get how to do the legend.

My code:

from pylab import figure, show, rand
from matplotlib.patches import Ellipse

NUM = 3

ells = [Ellipse(xy=rand(2)*10, width=rand(), height=rand(), angle=rand()*360)
        for i in range(NUM)]

fig = figure()
ax = fig.add_subplot(111, aspect='equal')
for e in ells:
    ax.add_artist(e)
    e.set_clip_box(ax.bbox)
    e.set_alpha(rand())
    e.set_facecolor(rand(3))

ax.set_xlim(0, 10)
ax.set_ylim(0, 10)

show()

Answer 1

In this case, you'll need to either specify the legend's artists and labels manually or use ax.add_patch instead of ax.add_artist.

---

legend checks a few particular lists of artists to decide what to add. Things like ax.lines, ax.collections, ax.patches, etc.

ax.add_artist is a low-level call for any type of artist. It's often used to add things that you wouldn't want in a legend. However, the add_<foo> variants add the artist using add_artist, but then append it to the appropriate list. Therefore, using ax.add_patch will append the artist to ax.patches, which legend will then check.

Alternatively, you can manually specify a list of artists and a list of labels to ax.legend to override what it automatically checks for.

---

In other words, you'll either need to call something similar to:

ax.legend(ells, ['label1', 'label2', 'label3'])

or do:

for i, e in enumerate(ells):
    ax.add_patch(e)
    e.set(clip_box=ax.bbox, alpha=rand(), facecolor=rand(3), 
          label='Ellipse{}'.format(i+1))
ax.legend()

---

As a full example of using ax.add_patch:

from numpy.random import rand
import matplotlib.pyplot as plt
from matplotlib.patches import Ellipse

NUM = 3
ellipse = lambda: Ellipse(rand(2)*10, rand(), rand(), rand()*360)
ells = [ellipse() for i in range(NUM)]

fig, ax = plt.subplots()

for i, e in enumerate(ells):
    ax.add_patch(e)
    e.set(clip_box=ax.bbox, alpha=rand(), facecolor=rand(3),
          label='Ellipse{}'.format(i+1))

ax.legend()
ax.set(xlim=[0, 10], ylim=[0, 10], aspect='equal')

plt.show()

And of manually specifying the artists and legend labels:

from numpy.random import rand
import matplotlib.pyplot as plt
from matplotlib.patches import Ellipse

NUM = 3
ellipse = lambda: Ellipse(rand(2)*10, rand(), rand(), rand()*360)
ells = [ellipse() for i in range(NUM)]

fig, ax = plt.subplots()

for e in ells:
    ax.add_artist(e)
    e.set(clip_box=ax.bbox, alpha=rand(), facecolor=rand(3))

ax.legend(ells, ['Ellipse{}'.format(i+1) for i in range(NUM)])
ax.set(xlim=[0, 10], ylim=[0, 10], aspect='equal')

plt.show()

Both produce identical results: