How do I use matplotlib autopct?

Question

I'd like to create a matplotlib pie chart which has the value of each wedge written on top of the wedge.

The documentation suggests I should use autopct to do this.

autopct: [ None | format string | format function ] If not None, is a string or function used to label the wedges with their numeric value. The label will be placed inside the wedge. If it is a format string, the label will be fmt%pct. If it is a function, it will be called.

Unfortunately, I'm unsure what this format string or format function is supposed to be.

Using this basic example below, how can I display each numerical value on top of its wedge?

plt.figure() values = [3, 12, 5, 8]  labels = ['a', 'b', 'c', 'd']  plt.pie(values, labels=labels) #autopct?? plt.show() 

Answer 1

You can do:

plt.pie(values, labels=labels, autopct=lambda p : '{:.2f}%  ({:,.0f})'.format(p,p * sum(values)/100)) 

Answer 2

autopct enables you to display the percent value using Python string formatting. For example, if autopct='%.2f', then for each pie wedge, the format string is '%.2f' and the numerical percent value for that wedge is pct, so the wedge label is set to the string '%.2f'%pct.

import matplotlib.pyplot as plt plt.figure() values = [3, 12, 5, 8]  labels = ['a', 'b', 'c', 'd']  plt.pie(values, labels=labels, autopct='%.2f') plt.show() 

yields

You can do fancier things by supplying a callable to autopct. To display both the percent value and the original value, you could do this:

import matplotlib.pyplot as plt  # make the pie circular by setting the aspect ratio to 1 plt.figure(figsize=plt.figaspect(1)) values = [3, 12, 5, 8]  labels = ['a', 'b', 'c', 'd']   def make_autopct(values):     def my_autopct(pct):         total = sum(values)         val = int(round(pct*total/100.0))         return '{p:.2f}%  ({v:d})'.format(p=pct,v=val)     return my_autopct  plt.pie(values, labels=labels, autopct=make_autopct(values)) plt.show() 

Again, for each pie wedge, matplotlib supplies the percent value pct as the argument, though this time it is sent as the argument to the function my_autopct. The wedge label is set to my_autopct(pct).