How Can I Add Group Labels to a Matplotlib Bar Chart Using Custom Code?

Using Custom Code to Label Bar Chart Groups

To add group labels to a bar chart, one approach that can be considered, especially if a native solution in matplotlib is not available, is to create a custom function. Here's how:

Custom Function for Group Labels

def label_group_bar(ax, data):
    # Prepare data
    groups = mk_groups(data)
    xy = groups.pop()
    x, y = zip(*xy)
    ly = len(y)

    # Create bar chart
    xticks = range(1, ly + 1)
    ax.bar(xticks, y, align='center')
    ax.set_xticks(xticks)
    ax.set_xticklabels(x)
    ax.set_xlim(.5, ly + .5)
    ax.yaxis.grid(True)

    # Add lines for group separation
    scale = 1. / ly
    for pos in range(ly + 1):
        add_line(ax, pos * scale, -.1)

    # Add labels below groups
    ypos = -.2
    while groups:
        group = groups.pop()
        pos = 0
        for label, rpos in group:
            lxpos = (pos + .5 * rpos) * scale
            ax.text(lxpos, ypos, label, ha='center', transform=ax.transAxes)
            add_line(ax, pos * scale, ypos)
            pos += rpos
        add_line(ax, pos * scale, ypos)
        ypos -= .1

Supporting Functions

To handle data preparation and line creation:

# Extract data groups and prepare for custom function
def mk_groups(data):
    ...

# Create vertical line in plot
def add_line(ax, xpos, ypos):
    ...

Usage

To use this solution:

# Import necessary modules
import matplotlib.pyplot as plt

# Sample data
data = ...

# Create plot and apply custom function
fig = plt.figure()
ax = fig.add_subplot(1,1,1)
label_group_bar(ax, data)
fig.subplots_adjust(bottom=0.3)
fig.savefig('label_group_bar_example.png')

Result

With this approach, you can add group labels to your bar chart, making the data visualization more informative.

Note: Alternative and possibly more optimized solutions are welcome for further consideration.

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