Plotting multiple lines in Matplotlib - Step-by-Step Execution

Concept Flow - Plotting multiple lines

Start matplotlib setup

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Create figure and axes

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Prepare data for line 1

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Plot line 1

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Prepare data for line 2

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Plot line 2

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Add labels and legend

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Show plot

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End

This flow shows how matplotlib prepares and draws multiple lines step-by-step on the same plot.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt

x = [1, 2, 3, 4]
y1 = [1, 4, 9, 16]
y2 = [2, 3, 5, 7]

plt.plot(x, y1, label='Squares')
plt.plot(x, y2, label='Primes')
plt.legend()
plt.show()

This code plots two lines on the same graph: squares and primes, then shows the plot with a legend.

Execution Table

Step	Action	Data Used	Plot State	Output
1	Import matplotlib.pyplot as plt	-	No plot created	Module loaded
2	Define x data	x = [1, 2, 3, 4]	No plot created	x data ready
3	Define y1 data	y1 = [1, 4, 9, 16]	No plot created	y1 data ready
4	Define y2 data	y2 = [2, 3, 5, 7]	No plot created	y2 data ready
5	Plot line 1 with label 'Squares'	x and y1	One line plotted	Line 1 drawn
6	Plot line 2 with label 'Primes'	x and y2	Two lines plotted	Line 2 drawn
7	Add legend	Labels from lines	Legend added	Legend visible
8	Show plot	All plotted lines and legend	Plot window opens	Graph displayed
9	End	-	Plot shown	Execution complete

💡 All lines plotted and displayed, program ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
x	undefined	[1, 2, 3, 4]	[1, 2, 3, 4]	[1, 2, 3, 4]	[1, 2, 3, 4]
y1	undefined	undefined	[1, 4, 9, 16]	[1, 4, 9, 16]	[1, 4, 9, 16]
y2	undefined	undefined	undefined	[2, 3, 5, 7]	[2, 3, 5, 7]
plot state	empty	empty	empty	empty	two lines with legend

Key Moments - 3 Insights

Why do we call plt.plot() twice instead of once?

Each plt.plot() call adds one line to the plot. Calling it twice draws two separate lines, as shown in steps 5 and 6 of the execution_table.

What does plt.legend() do and why is it called after plotting lines?

plt.legend() collects labels from all plotted lines and shows a legend box. It must be called after plotting lines to know their labels, as in step 7.

Why doesn't the plot show until plt.show() is called?

plt.show() opens the plot window and renders the graph. Before this, lines are prepared but not displayed, as seen in step 8.

Visual Quiz - 3 Questions

Look at the execution_table at step 6. How many lines are plotted after this step?

A Two lines

B One line

C No lines

D Three lines

💡 Hint

Check the 'Plot State' column at step 6 in the execution_table.

According to variable_tracker, what is the value of y2 after step 4?

A [1, 4, 9, 16]

B [2, 3, 5, 7]

C [1, 2, 3, 4]

D undefined

💡 Hint

Look at the 'y2' row and 'After Step 4' column in variable_tracker.

If we remove plt.legend(), what will happen to the plot output?

A Lines will not be drawn

B Plot will not open

C Plot will show but without legend

D An error will occur

💡 Hint

Refer to step 7 and 8 in execution_table about legend and plot display.

Concept Snapshot

Plotting multiple lines with matplotlib:
- Use plt.plot(x, y, label='name') for each line
- Call plt.legend() to show labels
- Call plt.show() to display the plot
- Each plt.plot() adds a new line
- Lines share the same axes by default

Full Transcript

This example shows how to plot multiple lines using matplotlib in Python. First, we import matplotlib.pyplot as plt. Then we prepare data lists for x and two y sets. We call plt.plot() twice, once for each line, passing x and y data with labels. After plotting lines, we call plt.legend() to add a legend box that shows line labels. Finally, plt.show() opens the plot window displaying both lines and the legend. Variables x, y1, and y2 hold the data arrays. The plot state changes from empty to having one line, then two lines, then the legend. This step-by-step process helps beginners see how multiple lines are added and displayed on the same graph.