Adding additional x axis labels betwen current labels and graph

Question

Consider the sample matplotlib gives for grouped bar charts seen here: https://matplotlib.org/stable/gallery/lines_bars_and_markers/barchart.html

With the code:

import matplotlib.pyplot as plt
import numpy as np


labels = ['G1', 'G2', 'G3', 'G4', 'G5']
men_means = [20, 34, 30, 35, 27]
women_means = [25, 32, 34, 20, 25]

x = np.arange(len(labels))  # the label locations
width = 0.35  # the width of the bars

fig, ax = plt.subplots()
rects1 = ax.bar(x - width/2, men_means, width, label='Men')
rects2 = ax.bar(x + width/2, women_means, width, label='Women')
ax.set_ylabel('Scores')
ax.set_title('Scores by group and gender')
ax.set_xticks(x, labels)
ax.legend()

ax.bar_label(rects1, padding=3)
ax.bar_label(rects2, padding=3)

fig.tight_layout()

plt.show()

they produce the following graph:

If I wanted to add the x-axis labels "Men" and "Women" in between G1, G2, G3, G4, and G5 and the bars, for each of the 5 measurements, how would I modify this code?

Answer 1

Based on the link @Stef provided, here's a working solution.

The patches attribute of your ax object contains the bars themselves, which are matplotlib.patches.Rectangle objects. If you loop through them, you can then use the get_x() and get_width() functions to get the x positions and bar widths, respectively. The x positions of each bar are actually the left side, so you need to add half the bar width to get the center.

sublabel_positions = []

# get_x() returns x position of bar's left side
# add 1/2 the bar width via get_width() to get the center
for bar in ax.patches:
    bar_center = (bar.get_x() + bar.get_width() / 2)
    sublabel_positions.append(bar_center)

As for the labels, since you defined rects1 before rects2 (the men values, then the women values), you want the list of labels to be 5 Men strings, followed by 5 Women strings:

sublabels = ['Men', 'Men', 'Men', 'Men', 'Men', 'Women', 'Women', 'Women', 'Women', 'Women']

# or you could do:
# sublabels = ['Men'] * 5 + ['Women'] * 5

Then you could add the extra x-ticks and labels with a single line shown below. The major x-ticks are your G1 thru G5, so you want to set minor=True here.

ax.set_xticks(sublabel_positions, sublabels, minor=True)

To make it look better, I ended up specifying a larger figure size, padding the major x-tick labels so they don't overlap, and turning off the major x-ticks, and I get this.

x = np.arange(len(labels))  # the label locations
width = 0.35  # the width of the bars

fig, ax = plt.subplots(figsize=(8, 6))
rects1 = ax.bar(x - width/2, men_means, width, label='Men')
rects2 = ax.bar(x + width/2, women_means, width, label='Women')

# Add some text for labels, title and custom x-axis tick labels, etc.
ax.set_ylabel('Scores')
ax.set_title('Scores by group and gender')
ax.set_xticks(x, labels)
ax.legend()

ax.bar_label(rects1, padding=3)
ax.bar_label(rects2, padding=3)

ax.set_xticks(sublabel_positions, sublabels, minor=True)
ax.tick_params(axis='x', which='major', pad=15)
ax.tick_params(axis='x', which='major', bottom=False)

fig.tight_layout()

plt.show()

(I'm sorry I don't have enough reputation to paste images yet)

Finished barplot with 2 series and stacked major/minor x-tick labels

As an aside, if you run ax.get_xticklabels(minor=True), you'll see what I mean about the order of bars being plotted—men first, then women:

>>> ax.get_xticklabels(minor=True)

[Text(-0.175, 0, 'Men'),
 Text(0.825, 0, 'Men'),
 Text(1.825, 0, 'Men'),
 Text(2.825, 0, 'Men'),
 Text(3.825, 0, 'Men'),
 Text(0.175, 0, 'Women'),
 Text(1.175, 0, 'Women'),
 Text(2.175, 0, 'Women'),
 Text(3.175, 0, 'Women'),
 Text(4.175, 0, 'Women')]