Stacked Bar Chart with Centered Labels

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Question :

Stacked Bar Chart with Centered Labels

I’m trying to “robustly” center the data labels in a stacked bar chart. A simple code 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()

Asked By: Bruno

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Answer #1:

Why you wrote va="bottom"? You have to use va="center".
enter image description here

Answered By: Serenity

Answer #2:

  • The following method is more succinct, and more easily scales with the number of columns.
  • Putting the data into a pandas.DataFrame is the easiest way to plot a stacked bar plot.
  • Using pandas.DataFrame.plot.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.
  • 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}%'

Imports

import pandas as pd
import matplotlib.pyplot as plt

Data

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

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()

enter image description here

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

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