Before-after comparison plots in Matplotlib - Step-by-Step Execution

Concept Flow - Before-after comparison plots

Load Data

↓

Identify Before and After

↓

Choose Plot Type

↓

Create Plot

↓

Display Comparison

↓

Interpret Results

The flow shows loading data, separating before and after values, choosing a plot style, creating the plot, displaying it, and interpreting the visual comparison.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
before = [5, 7, 8, 6]
after = [6, 9, 7, 8]
plt.plot(before, label='Before')
plt.plot(after, label='After')
plt.legend()
plt.show()

This code plots two lines showing values before and after an event for easy visual comparison.

Execution Table

Step	Action	Variable/Plot State	Output/Result
1	Import matplotlib.pyplot as plt	plt module ready	No output
2	Define before = [5,7,8,6]	before = [5,7,8,6]	No output
3	Define after = [6,9,7,8]	after = [6,9,7,8]	No output
4	Plot before line	Line for before plotted	Line graph with before values
5	Plot after line	Line for after plotted	Line graph with before and after lines
6	Add legend	Legend with 'Before' and 'After'	Legend displayed on plot
7	Show plot	Plot window opens	Visual comparison of before and after lines
8	End	Plot displayed	Execution stops

💡 Plot displayed and program ends

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
before	undefined	[5,7,8,6]	[5,7,8,6]	[5,7,8,6]
after	undefined	undefined	[6,9,7,8]	[6,9,7,8]
plt	undefined	module imported	module imported	module imported

Key Moments - 3 Insights

Why do we plot both 'before' and 'after' on the same graph?

Plotting both on the same graph lets us visually compare changes directly, as shown in execution_table steps 4 and 5.

What does plt.legend() do in the plot?

plt.legend() adds labels to the lines so we know which is 'Before' and which is 'After', as seen in step 6.

Why do we call plt.show() at the end?

plt.show() opens the plot window so we can see the graph; without it, the plot won't display, as in step 7.

Visual Quiz - 3 Questions

Look at the execution_table at step 5, what is plotted?

A Only the 'before' line

B Both 'before' and 'after' lines

C Only the 'after' line

D No lines plotted yet

💡 Hint

Check the 'Variable/Plot State' column at step 5 in execution_table

At which step does the legend get added to the plot?

A Step 6

B Step 5

C Step 4

D Step 7

💡 Hint

Look at the 'Action' column in execution_table for when plt.legend() is called

If we swapped the order of plotting 'before' and 'after', what would change in the plot?

A The plot would show only one line

B Nothing would change visually

C The lines would swap colors and legend order

D The plot would not display

💡 Hint

Consider how matplotlib assigns colors and legend labels in the order of plotting

Concept Snapshot

Before-after comparison plots:
- Plot 'before' and 'after' data on the same graph
- Use plt.plot() for each dataset
- Add plt.legend() to label lines
- Call plt.show() to display
- Helps visually compare changes over time

Full Transcript

This visual execution shows how to create before-after comparison plots using matplotlib in Python. First, we import the plotting library. Then, we define two lists: 'before' and 'after' values. We plot the 'before' data as a line, then plot the 'after' data on the same graph. Adding a legend labels each line for clarity. Finally, plt.show() displays the plot window so we can see the comparison visually. This step-by-step trace helps beginners understand how each command changes the plot state and why the order matters.