Before-after comparison plots in Matplotlib - Mini Project: Build & Apply

Before-After Comparison Plots

📖 Scenario: You work in a health clinic. You want to compare patients' blood pressure before and after a treatment.

🎯 Goal: Create a simple plot that shows blood pressure values before and after treatment for each patient. This helps to see if the treatment worked.

📋 What You'll Learn

Create a dictionary with patient names and their blood pressure before treatment

Create a dictionary with patient names and their blood pressure after treatment

Use matplotlib to plot before and after values side by side for each patient

Label the plot clearly with title, axis labels, and legend

💡 Why This Matters

🌍 Real World

Before-after comparison plots are used in health, marketing, and many fields to show changes clearly.

💼 Career

Data scientists often create comparison plots to communicate results of experiments or treatments.

1

Create blood pressure data dictionaries

Create a dictionary called bp_before with these exact entries: 'Anna': 140, 'Ben': 130, 'Cara': 150, 'Dan': 135. Then create another dictionary called bp_after with these exact entries: 'Anna': 130, 'Ben': 125, 'Cara': 140, 'Dan': 128.

Matplotlib

# Create the bp_before and bp_after dictionaries with exact values
# Your code here

Hint

Use curly braces {} to create dictionaries with the exact patient names and values.

2

Prepare patient names list for plotting

Create a list called patients that contains the patient names in this exact order: 'Anna', 'Ben', 'Cara', 'Dan'.

Matplotlib

bp_before = {'Anna': 140, 'Ben': 130, 'Cara': 150, 'Dan': 135}
bp_after = {'Anna': 130, 'Ben': 125, 'Cara': 140, 'Dan': 128}
# Create the patients list with exact names in order
# Your code here

Hint

Use square brackets [] to create a list with the patient names in the given order.

3

Plot before and after blood pressure values

Import matplotlib.pyplot as plt. Create two lists called before_values and after_values that contain the blood pressure values from bp_before and bp_after for each patient in patients. Then use plt.bar to plot the before values shifted left by 0.2 and after values shifted right by 0.2 on the x-axis positions. Use patients as x-axis labels.

Matplotlib

bp_before = {'Anna': 140, 'Ben': 130, 'Cara': 150, 'Dan': 135}
bp_after = {'Anna': 130, 'Ben': 125, 'Cara': 140, 'Dan': 128}
patients = ['Anna', 'Ben', 'Cara', 'Dan']
# Import matplotlib.pyplot as plt
# Create before_values and after_values lists
# Plot bars for before and after values with shifts
# Your code here

Hint

Use list comprehension to get values for each patient. Use plt.bar twice with shifted x positions for before and after bars.

4

Add labels and show the plot

Add a title 'Blood Pressure Before and After Treatment' using plt.title. Label the x-axis as 'Patients' and y-axis as 'Blood Pressure' using plt.xlabel and plt.ylabel. Add a legend with plt.legend(). Set the x-axis ticks to range(len(patients)) with labels patients using plt.xticks. Finally, display the plot with plt.show(). This will show the before and after blood pressure comparison clearly.

Matplotlib

import matplotlib.pyplot as plt

bp_before = {'Anna': 140, 'Ben': 130, 'Cara': 150, 'Dan': 135}
bp_after = {'Anna': 130, 'Ben': 125, 'Cara': 140, 'Dan': 128}
patients = ['Anna', 'Ben', 'Cara', 'Dan']

before_values = [bp_before[patient] for patient in patients]
after_values = [bp_after[patient] for patient in patients]

x = range(len(patients))
plt.bar([pos - 0.2 for pos in x], before_values, width=0.4, label='Before')
plt.bar([pos + 0.2 for pos in x], after_values, width=0.4, label='After')
# Add title, axis labels, legend, x-ticks, and show the plot
# Your code here

Hint

Use plt.title, plt.xlabel, plt.ylabel, plt.legend, plt.xticks, and plt.show to complete the plot.

Practice

1. What is the main purpose of a before-after comparison plot in matplotlib?

easy

A. To visually compare data from two different time points

B. To show the distribution of a single dataset

C. To display the correlation between two variables

D. To create a 3D surface plot

Solution

1. Step 1: Understand the concept of before-after plots

   Before-after plots are used to compare data points from two different times or conditions to see changes.

2. Step 2: Identify the correct purpose

   Among the options, only To visually compare data from two different time points describes comparing data from two time points, which matches the before-after plot purpose.

3. Final Answer:

   To visually compare data from two different time points -> Option A

4. Quick Check:

   Before-after plots = compare two time points [OK]

Hint: Before-after plots compare two sets of data visually [OK]

Common Mistakes:

• Confusing before-after plots with distribution plots
• Thinking they show correlation instead of change
• Assuming they create 3D plots

2. Which of the following is the correct way to plot two sets of data side-by-side for before-after comparison using matplotlib?

easy

A. plt.plot(before_data); plt.plot(after_data)

B. plt.bar([0,1], before_data); plt.bar([0,1], after_data)

C. plt.bar([0,1], before_data); plt.bar([1,2], after_data)

D. plt.bar([1,2], [before_data, after_data])

Solution

1. Step 1: Understand bar plot positioning

   To show before and after side-by-side, bars must not overlap. Using different x positions for before and after data avoids overlap.

2. Step 2: Analyze options for correct bar positions

   plt.bar([0,1], before_data); plt.bar([1,2], after_data) places before_data at positions 0 and 1, and after_data at 1 and 2, so bars for the same category are side-by-side without overlap.

3. Final Answer:

   plt.bar([0,1], before_data); plt.bar([1,2], after_data) -> Option C

4. Quick Check:

   Side-by-side bars need different x positions [OK]

Hint: Use different x positions to avoid bar overlap [OK]

Common Mistakes:

• Plotting bars at same x positions causing overlap
• Using plt.plot instead of plt.bar for categorical data
• Passing data incorrectly as list of lists

3. What will be the output of this code snippet?

import matplotlib.pyplot as plt
before = [5, 7]
after = [8, 6]
plt.plot([1, 2], before, label='Before')
plt.plot([1, 2], after, label='After')
plt.legend()
plt.show()

medium

A. An error because plt.plot cannot take two lists

B. A bar chart comparing before and after data

C. A scatter plot with points at (1,5), (2,7), (1,8), (2,6)

D. Two overlapping line plots showing before and after data

Solution

1. Step 1: Understand plt.plot with x and y lists

   plt.plot([1, 2], before) plots points (1,5) and (2,7) connected by a line. Similarly for after data.

2. Step 2: Identify plot type and legend

   Two line plots will appear overlapping on the same axes with labels 'Before' and 'After'. No error occurs.

3. Final Answer:

   Two overlapping line plots showing before and after data -> Option D

4. Quick Check:

   plt.plot with x,y lists = line plot [OK]

Hint: plt.plot(x, y) draws lines connecting points [OK]

Common Mistakes:

• Thinking plt.plot creates bar charts
• Expecting scatter plot without plt.scatter
• Assuming plt.plot with two lists causes error

4. Identify the error in this code for before-after bar plot:

import matplotlib.pyplot as plt
before = [3, 4]
after = [5, 6]
plt.bar([0, 1], before)
plt.bar([0, 1], after)
plt.show()

medium

A. plt.show() is missing

B. Bars for before and after overlap at same positions

C. before and after lists must be same length

D. plt.bar requires three arguments

Solution

1. Step 1: Check bar positions

   Both before and after bars are plotted at positions 0 and 1, causing them to overlap and hide one another.

2. Step 2: Identify correct fix

   To avoid overlap, after bars should be shifted to different x positions, e.g., [0.3, 1.3].

3. Final Answer:

   Bars for before and after overlap at same positions -> Option B

4. Quick Check:

   Same x positions cause bar overlap [OK]

Hint: Shift bars on x-axis to avoid overlap [OK]

Common Mistakes:

• Thinking plt.bar needs 3 arguments
• Ignoring bar overlap issue
• Assuming plt.show() is missing

5. You have sales data before and after a marketing campaign for 3 products: before = [100, 150, 200], after = [120, 180, 210]. How would you create a clear before-after bar plot with labels and legend in matplotlib?

hard

A. Use plt.bar with shifted x positions for before and after, add labels and legend

B. Plot before and after using plt.plot without labels

C. Use plt.scatter for both datasets on same x positions

D. Plot only after data as a bar chart

Solution

1. Step 1: Plan bar positions and labels

   To compare before and after clearly, plot bars side-by-side with shifted x positions, e.g., before at [0,1,2], after at [0.3,1.3,2.3]. Add x-axis labels for products.

2. Step 2: Add legend and labels for clarity

   Use plt.legend() to distinguish before and after bars, and plt.xlabel/plt.ylabel for axis labels.

3. Final Answer:

   Use plt.bar with shifted x positions for before and after, add labels and legend -> Option A

4. Quick Check:

   Shift bars + labels + legend = clear before-after plot [OK]

Hint: Shift bars and add legend for clear comparison [OK]

Common Mistakes:

• Plotting bars at same positions causing confusion
• Skipping labels and legend
• Using scatter plot instead of bar plot