Basic plt.plot usage in Matplotlib - Step-by-Step Execution

Concept Flow - Basic plt.plot usage

Import matplotlib.pyplot as plt

↓

Prepare data: x and y lists

↓

Call plt.plot(x, y)

↓

Plot line graph appears

↓

Call plt.show() to display plot

↓

Graph window opens

This flow shows how to import matplotlib, prepare data, plot it using plt.plot, and display the graph.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3, 4]
y = [10, 20, 25, 30]
plt.plot(x, y)
plt.show()

This code plots points (1,10), (2,20), (3,25), (4,30) connected by lines and shows the graph.

Execution Table

Step	Action	Data State	Plot State	Output
1	Import matplotlib.pyplot as plt	No data yet	No plot	Module loaded
2	Create x = [1, 2, 3, 4]	x = [1, 2, 3, 4]	No plot	List x created
3	Create y = [10, 20, 25, 30]	y = [10, 20, 25, 30]	No plot	List y created
4	Call plt.plot(x, y)	x and y unchanged	Line plot prepared	Plot line connecting points
5	Call plt.show()	Data unchanged	Plot displayed	Graph window opens showing line plot

💡 Plot displayed and program waits until window is closed

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
x	undefined	[1, 2, 3, 4]	[1, 2, 3, 4]	[1, 2, 3, 4]
y	undefined	undefined	[10, 20, 25, 30]	[10, 20, 25, 30]

Key Moments - 2 Insights

Why do we need to call plt.show() after plt.plot()?

plt.plot() prepares the plot but does not display it. plt.show() opens the window to display the graph, as shown in execution_table step 5.

What happens if x and y lists have different lengths?

Matplotlib will raise an error because it needs matching x and y points to plot lines. This is not shown here but is important to keep lists equal length.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of the plot after step 4?

A Line plot prepared connecting points

B No plot prepared yet

C Plot displayed on screen

D Plot data cleared

💡 Hint

Check the 'Plot State' column at step 4 in the execution_table

At which step does the graph window open to show the plot?

A Step 2

B Step 3

C Step 5

D Step 4

💡 Hint

Look at the 'Output' column in execution_table for when the graph window opens

If we change y to [5, 15, 20, 25], what will happen to the plot?

A The plot will not change

B The plot will show points with new y values connected by lines

C An error will occur because y changed

D The plot will show points with old y values

💡 Hint

Variable tracker shows y values affect the plot points connected by plt.plot

Concept Snapshot

Basic plt.plot usage:
- Import matplotlib.pyplot as plt
- Prepare x and y data lists
- Call plt.plot(x, y) to create line plot
- Call plt.show() to display the plot window
- x and y must be same length
- Plot connects points (x[i], y[i]) with lines

Full Transcript

This visual execution shows how to use matplotlib's plt.plot function. First, we import matplotlib.pyplot as plt. Then we create two lists x and y with numbers representing points on the x and y axes. Calling plt.plot(x, y) prepares a line plot connecting these points but does not display it yet. Finally, plt.show() opens a window showing the graph. Variables x and y hold the data points. The plot state changes from no plot to line plot prepared, then to plot displayed. Remember, plt.show() is needed to see the graph. Also, x and y must have the same length to avoid errors.

Practice

1. What does the plt.plot(x, y) function do in matplotlib?

easy

A. It creates a bar chart from x and y data.

B. It clears the current plot.

C. It saves the plot as an image file.

D. It draws a line connecting points defined by x and y data.

Solution

1. Step 1: Understand plt.plot function

   The plt.plot(x, y) function connects points defined by x and y coordinates with lines.

2. Step 2: Differentiate from other functions

   Other options describe different matplotlib functions: bar chart, saving, or clearing plots, which are not done by plt.plot.

3. Final Answer:

   It draws a line connecting points defined by x and y data. -> Option D

4. Quick Check:

   plt.plot connects points with lines [OK]

Hint: Remember: plt.plot connects points with lines [OK]

Common Mistakes:

• Confusing plt.plot with bar chart functions
• Thinking plt.plot saves images
• Assuming plt.plot clears plots

2. Which of the following is the correct syntax to plot y versus x using matplotlib?

easy

A. plt.plot(x = y)

B. plt.plot(y, x)

C. plt.plot(x, y)

D. plt.plot(y)

Solution

1. Step 1: Check the order of parameters in plt.plot

   The first argument is x data, the second is y data, so plt.plot(x, y) is correct.

2. Step 2: Identify incorrect syntax

   Options A uses invalid keyword argument, B reverses x and y, and D misses x data.

3. Final Answer:

   plt.plot(x, y) -> Option C

4. Quick Check:

   Correct order is x then y [OK]

Hint: Plot syntax is plt.plot(x, y) with x first [OK]

Common Mistakes:

• Swapping x and y arguments
• Using invalid keyword arguments
• Providing only one argument without x

3. What will be the output of this code?

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [4, 5, 6]
plt.plot(x, y)
plt.show()

medium

A. No plot will be shown because plt.show() is missing.

B. A line graph connecting points (1,4), (2,5), and (3,6).

C. A bar chart with bars at x=1,2,3 with heights 4,5,6.

D. A scatter plot of points (1,4), (2,5), and (3,6).

Solution

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

   The code plots points (1,4), (2,5), and (3,6) connected by lines.

2. Step 2: Confirm plt.show() displays the plot

   The plt.show() command displays the line graph on screen.

3. Final Answer:

   A line graph connecting points (1,4), (2,5), and (3,6). -> Option B

4. Quick Check:

   plt.plot + plt.show = line graph [OK]

Hint: plt.plot draws lines; plt.show displays them [OK]

Common Mistakes:

• Confusing plt.plot with scatter plot
• Thinking plt.plot creates bar charts
• Forgetting plt.show to display plot

4. Identify the error in this code snippet:

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [4, 5]
plt.plot(x, y)
plt.show()

medium

A. x and y lists have different lengths causing an error.

B. plt.plot requires only one argument, so two are invalid.

C. plt.show() is missing to display the plot.

D. No error; the plot will display correctly.

Solution

1. Step 1: Check lengths of x and y lists

   x has 3 elements, y has 2 elements; they must be equal length for plt.plot.

2. Step 2: Understand plt.plot requirements

   plt.plot needs x and y lists of same length to pair points; mismatch causes error.

3. Final Answer:

   x and y lists have different lengths causing an error. -> Option A

4. Quick Check:

   Equal length x and y needed [OK]

Hint: x and y must have same length for plt.plot [OK]

Common Mistakes:

• Ignoring length mismatch of x and y
• Thinking plt.plot accepts different length lists
• Assuming plt.show() missing causes error

5. You want to plot two lines on the same graph: y = x and y = x² for x values 0 to 3. Which code correctly does this?

hard

A. plt.plot([0,1,2,3], [0,1,2,3])
plt.plot([0,1,2,3], [0,1,4,9])
plt.show()

B. plt.plot([0,1,2,3], [0,1,4,9])
plt.show()
plt.plot([0,1,2,3], [0,1,2,3])

C. plt.plot([0,1,2,3], [0,1,2,3], [0,1,4,9])
plt.show()

D. plt.plot([0,1,2,3], [0,1,2,3])
plt.show()
plt.plot([0,1,2,3], [0,1,4,9])

Solution

1. Step 1: Plot first line y = x

   Use plt.plot([0,1,2,3], [0,1,2,3]) to plot y = x.

2. Step 2: Plot second line y = x² on same graph

   Use plt.plot([0,1,2,3], [0,1,4,9]) to plot y = x² on same figure before plt.show().

3. Step 3: Call plt.show() once after both plots

   Calling plt.show() displays both lines together.

4. Final Answer:

   plt.plot([0,1,2,3], [0,1,2,3]) plt.plot([0,1,2,3], [0,1,4,9]) plt.show() -> Option A

5. Quick Check:

   Multiple plt.plot calls before plt.show() [OK]

Hint: Call plt.plot twice, then plt.show once [OK]

Common Mistakes:

• Calling plt.show() before all plots
• Trying to plot two lines in one plt.plot call incorrectly
• Plotting lines in wrong order (does not affect but confuses)