Stacked Bar Chart with Centered Labels

I’m trying to “robustly” center the data labels in a stacked bar chart. A simple code example and the result are given below. As you can see, the data labels aren’t really centered in all rectangles. What am I missing?

import numpy as np
import matplotlib.pyplot as plt

A = [45, 17, 47]
B = [91, 70, 72]

fig = plt.figure(facecolor="white")

ax = fig.add_subplot(1, 1, 1)
bar_width = 0.5
bar_l = np.arange(1, 4)
tick_pos = [i + (bar_width / 2) for i in bar_l]

ax1 = ax.bar(bar_l, A, width=bar_width, label="A", color="green")
ax2 = ax.bar(bar_l, B, bottom=A, width=bar_width, label="B", color="blue")
ax.set_ylabel("Count", fontsize=18)
ax.set_xlabel("Class", fontsize=18)
ax.legend(loc="best")
plt.xticks(tick_pos, ["C1", "C2", "C3"], fontsize=16)
plt.yticks(fontsize=16)

for r1, r2 in zip(ax1, ax2):
    h1 = r1.get_height()
    h2 = r2.get_height()
    plt.text(r1.get_x() + r1.get_width() / 2., h1 / 2., "%d" % h1, ha="center", va="bottom", color="white", fontsize=16, fontweight="bold")
    plt.text(r2.get_x() + r2.get_width() / 2., h1 + h2 / 2., "%d" % h2, ha="center", va="bottom", color="white", fontsize=16, fontweight="bold")

plt.show()

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Answers:

Method 1

Imports & Test DataFrame

Updated for matplotlib v3.4.2

Annotation Resources – from matplotlib v3.4.2

Original Answer

Plot

Method 2

Answers:

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Method 1

The following method is more succinct, and easily scales.
Putting the data into a pandas.DataFrame is the easiest way to plot a stacked bar plot.
Using pandas.DataFrame.plot.bar(stacked=True), or pandas.DataFrame.plot(kind='bar', stacked=True), is the easiest way to plot a stacked bar plot.
- This method returns a matplotlib.axes.Axes or a numpy.ndarray of them.
Since seaborn is just a high-level API for matplotlib, these solutions also work with seaborn plots, as shown in How to annotate a seaborn barplot with the aggregated value.

Imports & Test DataFrame

import pandas as pd
import matplotlib.pyplot as plt

A = [45, 17, 47]
B = [91, 70, 72]
C = [68, 43, 13]

# pandas dataframe
df = pd.DataFrame(data={'A': A, 'B': B, 'C': C})
df.index = ['C1', 'C2', 'C3']

     A   B   C
C1  45  91  68
C2  17  70  43
C3  47  72  13

Updated for `matplotlib v3.4.2`

Use matplotlib.pyplot.bar_label
- Will automatically center the values in the bar.
- See this answer for additional details about .bar_label()
See the matplotlib: Bar Label Demo page for additional formatting options.
Tested with pandas v1.2.4, which is using matplotlib as the plot engine.
If some sections of the bar plot will be zero, see my answer, which shows how to customize the labels for .bar_label().
ax.bar_label(c, fmt='%0.0f', label_type='center') will change the number format to show no decimal places, if needed.

ax = df.plot(kind='bar', stacked=True, figsize=(8, 6), rot=0, xlabel='Class', ylabel='Count')
for c in ax.containers:

    # Optional: if the segment is small or 0, customize the labels
    labels = [v.get_height() if v.get_height() > 0 else '' for v in c]
    
    # remove the labels parameter if it's not needed for customized labels
    ax.bar_label(c, labels=labels, label_type='center')

Annotation Resources – from `matplotlib v3.4.2`

Original Answer

Using the .patches method unpacks a list of matplotlib.patches.Rectangle objects, one for each of the sections of the stacked bar.
- Each .Rectangle has methods for extracting the various values that define the rectangle.
- Each .Rectangle is in order from left to right, and bottom to top, so all the .Rectangle objects, for each level, appear in order, when iterating through .patches.
The labels are made using an f-string, label_text = f'{height}', so any additional text can be added as needed, such as label_text = f'{height}%'
- label_text = f'{height:0.0f}' will display numbers with no decimal places.

Plot

plt.style.use('ggplot')

ax = df.plot(stacked=True, kind='bar', figsize=(12, 8), rot='horizontal')

# .patches is everything inside of the chart
for rect in ax.patches:
    # Find where everything is located
    height = rect.get_height()
    width = rect.get_width()
    x = rect.get_x()
    y = rect.get_y()
    
    # The height of the bar is the data value and can be used as the label
    label_text = f'{height}'  # f'{height:.2f}' to format decimal values
    
    # ax.text(x, y, text)
    label_x = x + width / 2
    label_y = y + height / 2

    # plot only when height is greater than specified value
    if height > 0:
        ax.text(label_x, label_y, label_text, ha='center', va='center', fontsize=8)
    
ax.legend(bbox_to_anchor=(1.05, 1), loc='upper left', borderaxespad=0.)    
ax.set_ylabel("Count", fontsize=18)
ax.set_xlabel("Class", fontsize=18)
plt.show()

To plot a horizontal bar:
- kind='barh'
- label_text = f'{width}'
- if width > 0:
Attribution: jsoma/chart.py

Method 2

Why you wrote va="bottom"? You have to use va="center".

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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