修改Matplotlib PatchCollection中的特定补丁

Question

Let's assume that I use a third-party function magic_plot(data, ax) that adds a collection of patches to the axes based on the provided data. Let's further assume that I want to change the color of a specific patch that that has been added. How do I do that?

from numpy.random import rand

from matplotlib.collections import PatchCollection
from matplotlib.patches import Circle
import matplotlib.pyplot as plt

def magic_plot(data, ax):
    """
    A third-party plotting function, not modifiable by end-users. 
    """
    lst = []
    for i in range(10):
        patch = Circle((rand(), rand()), rand())
        lst.append(patch)
    collection = PatchCollection(lst)
    ax.add_collection(collection)

ax = plt.gca()
data = rand(100)

# create the plot:
magic_plot(data, ax)

# obtain the PatchCollection created by magic_plot():
collection = ax.collections[0]

As shown above, I can retrieve the collection from the axes from ax.collections , but how do I proceed from here?

I assume that I need to access the list of patches that are stored in this PatchCollection object. However, in an answer to a similar question "matplotlib change a Patch in PatchCollection" , it has been suggested to use a subclass of PatchCollection that tracks the patches that are added to it in a public list attribute, but given that the collection is created in a third-party function, this solution is not an option in my case.

Answer 1

Updating the shape of the patches is hardly possible, as shown in the linked question. However, here you want to change the color of a patch. This should be much easier.

In the concrete example from the question, it will be achieved simply by setting the facecolor of the collection.

import numpy as np
np.random.seed(50)

from matplotlib.collections import PatchCollection
from matplotlib.patches import Circle
import matplotlib.pyplot as plt
import matplotlib.colors as mcolors

def magic_plot(data, ax):
    """
    A third-party plotting function, not modifiable by end-users. 
    """
    lst = []
    for i in range(10):
        patch = Circle((np.random.rand(), np.random.rand()), np.random.rand()/9)
        lst.append(patch)
    collection = PatchCollection(lst)
    ax.add_collection(collection)

ax = plt.gca()
data = np.random.rand(100)

# create the plot:
magic_plot(data, ax)
ax.autoscale()

# obtain the PatchCollection created by magic_plot():
collection = ax.collections[0]

n = len(collection.get_paths())
facecolors = collection.get_facecolors()
if len(facecolors) == 1 and n != 1:
    facecolors = np.array([facecolors[0]] * n)

# change the facecolor of the fourth patch (index=3)
facecolors[3] = mcolors.to_rgba("crimson")
collection.set_facecolor(facecolors)

plt.show()