How to Add Group Labels to Matplotlib Bar Charts?

Adding Group Labels to Bar Charts

When plotting data in the form of a bar chart using matplotlib, it is often desirable to distinguish between different groups of data. The data structure might resemble the following:

data = {'Room A':
           {'Shelf 1':
               {'Milk': 10,
                'Water': 20},
            'Shelf 2':
               {'Sugar': 5,
                'Honey': 6}
           },
        'Room B':
           {'Shelf 1':
               {'Wheat': 4,
                'Corn': 7},
            'Shelf 2':
               {'Chicken': 2,
                'Cow': 1}
           }
       }

The desired output, represented as an image, is:

[Image showing bar chart with groups labeled]

Implementing Group Labels

As there is no built-in solution for adding group labels in matplotlib, a custom implementation can be devised:

#!/usr/bin/env python

from matplotlib import pyplot as plt

def mk_groups(data):
    try:
        newdata = data.items()
    except:
        return

    thisgroup = []
    groups = []
    for key, value in newdata:
        newgroups = mk_groups(value)
        if newgroups is None:
            thisgroup.append((key, value))
        else:
            thisgroup.append((key, len(newgroups[-1])))
            if groups:
                groups = [g + n for n, g in zip(newgroups, groups)]
            else:
                groups = newgroups
    return [thisgroup] + groups

def add_line(ax, xpos, ypos):
    line = plt.Line2D([xpos, xpos], [ypos + .1, ypos],
                      transform=ax.transAxes, color='black')
    line.set_clip_on(False)
    ax.add_line(line)

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

    scale = 1. / ly
    for pos in xrange(ly + 1):  # change xrange to range for python3
        add_line(ax, pos * scale, -.1)
    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

if __name__ == '__main__':
    data = {'Room A':
               {'Shelf 1':
                   {'Milk': 10,
                    'Water': 20},
                'Shelf 2':
                   {'Sugar': 5,
                    'Honey': 6}
               },
            'Room B':
               {'Shelf 1':
                   {'Wheat': 4,
                    'Corn': 7},
                'Shelf 2':
                   {'Chicken': 2,
                    'Cow': 1}
               }
           }
    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')

The mk_groups function converts the data into a suitable format for creating the chart. add_line is responsible for adding vertical lines to the subplot at specified positions. The label_group_bar function generates the bar chart with the group labels underneath.

The result of this implementation is a bar chart with clearly labeled groups:

[Image showing bar chart with groups labeled]

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