Double donut chart in matplotlib

Question

Alright matplotlib afficionados, we know how to plot a donut chart, but what is better than a donut chart? A double-donut chart. Specifically: We have a set of elements that fall into disjoint categories and sub-categories of the first categorization. The donut chart should have slices for the categories in the outer ring and slices for the sub-categories in the inner ring, obviously aligned with the outer slices.

Is there any library that provides this or do we need to work this out here?

Answer 1

To obtain a double donut chart, you can plot as many pie charts in the same plot as you want. So the outer pie would have a width set to its wedges and the inner pie would have a radius that is less or equal 1-width.

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()
ax.axis('equal')
width = 0.3

cm = plt.get_cmap("tab20c")
cout = cm(np.arange(3)*4)
pie, _ = ax.pie([120,77,39], radius=1, labels=list("ABC"), colors=cout)
plt.setp( pie, width=width, edgecolor='white')

cin = cm(np.array([1,2,5,6,9,10]))
labels = list(map("".join, zip(list("aabbcc"),map(str, [1,2]*3))))
pie2, _ = ax.pie([60,60,37,40,29,10], radius=1-width, labels=labels,
                                      labeldistance=0.7, colors=cin)
plt.setp( pie2, width=width, edgecolor='white')
plt.show()

Note: I made this code also available in the matplotlib gallery as nested pie example.