Basic plt.plot usage in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What does this matplotlib code plot?

Consider the following Python code using matplotlib. What kind of plot will it produce?

Matplotlib

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

A A line graph connecting points (1,10), (2,20), (3,25), (4,30)

B A scatter plot with points at (1,10), (2,20), (3,25), (4,30)

C A bar chart with bars at x=1,2,3,4 with heights 10,20,25,30

D An empty plot with no data shown

💡 Hint

plt.plot by default draws lines connecting points.

The plt.plot function draws a line graph connecting the points given by x and y lists.

❓ data_output

intermediate

What is the y-value of the third point plotted?

Given this code snippet, what is the y-value of the third point plotted?

Matplotlib

import matplotlib.pyplot as plt
x = [0, 1, 2, 3]
y = [5, 15, 25, 35]
plt.plot(x, y)
plt.show()

A 25

B 15

C 35

D 5

💡 Hint

Indexing starts at 0, so the third point is at index 2.

The third point corresponds to index 2 in the y list, which is 25.

❓ visualization

advanced

Which option produces a red dashed line plot?

Which code snippet will produce a red dashed line connecting points (1,2), (2,4), (3,6)?

A plt.plot([1,2,3], [2,4,6], 'g-')

B plt.plot([1,2,3], [2,4,6], color='blue', linestyle='--')

C plt.plot([1,2,3], [2,4,6], 'r--')

D plt.plot([1,2,3], [2,4,6], linestyle=':')

💡 Hint

The format string 'r--' means red dashed line.

The 'r--' format string sets the line color to red and line style to dashed.

🔧 Debug

advanced

What error does this code raise?

What error will this code raise when executed?

Matplotlib

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

A TypeError: unsupported operand type(s)

B ValueError: x and y must have same first dimension

C IndexError: list index out of range

D No error, plot will display correctly

💡 Hint

x and y lists must be the same length for plt.plot.

The x list has length 3 but y has length 2, causing a ValueError.

🚀 Application

expert

How many lines will this code plot?

Given this code, how many separate lines will appear in the plot?

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3]
y1 = [2, 4, 6]
y2 = [1, 3, 5]
plt.plot(x, y1, x, y2)
plt.show()

A 3

B 1

C 0

D 2

💡 Hint

plt.plot can take multiple x,y pairs to plot multiple lines.

The code plots two lines: one for (x,y1) and one for (x,y2).

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)