Pie charts represent numerical proportions, where the arc length of each segment is proportional to the quantity it represents.

They are compact and can look very aesthetically pleasing, but they have been criticized as they can be hard to compare. Another property of pie charts that does not work in their best interest is that pie charts are presented in a specific angle (perspective) and segments use certain colors that can skew our perception and influence our conclusion about information presented.

What we will show here is different ways to use pie charts to present data.

Here, we create a so-called exploded pie chart:

from pylab import *

# make a square figure and axes
figure(1, figsize=(6,6))
ax = axes([0.1, 0.1, 0.8, 0.8])

# the slices will be ordered
# and plotted counter-clockwise.
labels = 'Spring', 'Summer', 'Autumn', 'Winter'

# fractions are either x/sum(x) or x if sum(x) <= 1
x = [15, 30, 45, 10]

# explode must be len(x) sequence or None
explode=(0.1, 0.1, 0.1, 0.1)

pie(x, explode=explode, labels=labels,
autopct='%1.1f%%', startangle=67)

title('Rainy days by season')

show()

Pie charts look best if they are inside a square figure and have square axes.

Fractions of the whole sum of the pie chart are defined as x/sum(x) or x if sum(x) <= 1. We get the explode effect by defining an explode sequence where each item represents the fraction of radius with which to offset each arc. We use the autopct parameter to format the labels that will be drawn inside the arcs; they can be a format string or a callable (function).

We can also use a Boolean shadow parameter to add a shadow effect to a pie chart.

If we don't specify startangle, the fractions will be ordered starting counterclockwise from the x axis (angle 0). If we specify 90 as the value of startangle, that will start the pie chart from the y axis.

This is the resulting pie chart: