Horizontal grouped bars in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of horizontal grouped bar chart code

What will be the output of this code snippet that creates a horizontal grouped bar chart using matplotlib?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

labels = ['A', 'B', 'C']
values1 = [5, 7, 3]
values2 = [6, 2, 4]

x = np.arange(len(labels))
width = 0.35

fig, ax = plt.subplots()
ax.barh(x - width/2, values1, width, label='Group 1')
ax.barh(x + width/2, values2, width, label='Group 2')
ax.set_yticks(x)
ax.set_yticklabels(labels)
ax.legend()
plt.show()

A A vertical grouped bar chart with bars for each label A, B, C stacked on top of each other

B A horizontal grouped bar chart with two bars side by side for each label A, B, C, showing values from values1 and values2

C A horizontal bar chart with bars for values1 only, no grouping

D A scatter plot with points at coordinates from values1 and values2

💡 Hint

Look at the use of barh and the x positions shifted by width/2

The code uses barh to create horizontal bars. The bars for values1 and values2 are shifted left and right by width/2 to group them side by side for each label.

❓ data_output

intermediate

Number of bars in horizontal grouped bar chart

Given this code for a horizontal grouped bar chart, how many bars will be drawn in total?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

labels = ['X', 'Y', 'Z', 'W']
values1 = [3, 5, 2, 4]
values2 = [1, 6, 3, 7]
values3 = [2, 4, 5, 1]

x = np.arange(len(labels))
width = 0.2

fig, ax = plt.subplots()
ax.barh(x - width, values1, width, label='Set 1')
ax.barh(x, values2, width, label='Set 2')
ax.barh(x + width, values3, width, label='Set 3')
ax.set_yticks(x)
ax.set_yticklabels(labels)
ax.legend()
plt.show()

A 4

B 7

C 3

D 12

💡 Hint

Count bars per group times number of groups

There are 4 labels and 3 sets of bars per label, so total bars = 4 * 3 = 12.

🔧 Debug

advanced

Identify the error in horizontal grouped bar chart code

What error will this code produce when trying to plot a horizontal grouped bar chart?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

labels = ['P', 'Q', 'R']
values1 = [4, 6, 5]
values2 = [7, 3, 8]

x = np.arange(len(labels))
width = 0.3

fig, ax = plt.subplots()
ax.barh(x, values1, width, label='First')
ax.barh(x + width, values2, width, label='Second')
ax.set_yticks(x)
ax.set_yticklabels(labels)
ax.legend()
plt.show()

A TypeError: unsupported operand type(s) for +: 'numpy.ndarray' and 'float'

B ValueError: x and height must have the same size

C No error, the chart displays correctly

D TypeError: unsupported operand type(s) for +: 'range' and 'float'

💡 Hint

Check the type of x and the operation x + width

x is a numpy.ndarray created by np.arange. NumPy arrays support broadcasting addition with floats, so x + width works fine and produces another ndarray. The bars abut perfectly without overlap or gap. No error occurs, and the chart displays correctly.

❓ visualization

advanced

Interpret the horizontal grouped bar chart visualization

You see a horizontal grouped bar chart with labels ['Jan', 'Feb', 'Mar'] on the y-axis. Each label has two bars side by side. The first bar is always longer than the second. What can you conclude about the data?

A The first group has higher values than the second group for all months

B The second group has higher values than the first group for all months

C Both groups have equal values for all months

D The chart shows stacked bars, not grouped bars

💡 Hint

Look at the length of bars for each group per label

In grouped bars, side by side bars represent different groups. Longer bars mean higher values. Since the first bar is longer for all months, the first group has higher values.

🚀 Application

expert

Calculate bar positions for horizontal grouped bar chart

You want to plot a horizontal grouped bar chart with 4 groups and 5 bars in each group. The bar height should be 0.15. What is the correct numpy expression to calculate the y positions for the bars in the third group (index 2)?

A np.arange(5) + (2 - 1.5) * 0.15

B np.arange(5) + (2 - 2) * 0.15

C np.arange(5) + (2 - 1.5) * 0.15 - 0.075

D np.arange(5) + 2 * 0.15

💡 Hint

The offset for group j in n=4 groups is (j - (n-1)/2 ) * height = (j - 1.5) * 0.15. The y in barh is the center position.

In horizontal grouped bar charts, y positions are the centers of the bars. For n_groups=4 and height=0.15, the centers are offset by (j - 1.5) * 0.15 to abut perfectly and center the group around the tick at np.arange(5). For j=2 (third group), offset = (2 - 1.5)*0.15 = 0.075.