Basic plt.plot usage in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

It creates a line plot by connecting points defined by the lists or arrays x and y on the x-axis and y-axis respectively.

beginner

How do you add labels to the x-axis and y-axis in matplotlib?

Use plt.xlabel('label') for the x-axis and plt.ylabel('label') for the y-axis to add descriptive labels.

beginner

What is the purpose of plt.show()?

It displays the plot window with all the current figures and plots. Without it, the plot might not appear in some environments.

beginner

How can you change the line style and color in plt.plot()?

You can add a format string like 'r--' where 'r' is red color and '--' is dashed line. For example, plt.plot(x, y, 'r--').

beginner

What happens if you call plt.plot() multiple times before plt.show()?

All plots are drawn on the same figure, allowing you to overlay multiple lines or data sets in one graph.

What does plt.plot([1, 2, 3], [4, 5, 6]) do?

A Creates a bar chart

B Plots points (4,1), (5,2), (6,3)

C Plots points (1,4), (2,5), (3,6) connected by lines

D Creates a scatter plot without lines

The first list is x values and the second is y values, connected by lines.

Which command adds a title to a matplotlib plot?

A plt.title('My Plot')

B plt.label('My Plot')

C plt.name('My Plot')

D plt.header('My Plot')

Use plt.title() to add a title to the plot.

How do you display the plot after creating it?

A plt.render()

B plt.display()

C plt.draw()

D plt.show()

plt.show() opens the window with the plot.

What does the format string 'g-.' mean in plt.plot(x, y, 'g-.')?

A Green dash-dot line

B Gray solid line

C Green dotted line

D Gold dashed line

'g' is green color, '-.' is dash-dot line style.

If you want to plot two lines on the same graph, what should you do?

A Call plt.show() twice

B Call plt.plot() twice before plt.show()

C Use plt.plot() only once with combined data

D Create two separate figures

Multiple plt.plot() calls add lines to the same figure.

Explain how to create a simple line plot with labels for x and y axes using matplotlib.

Think about the steps to draw and display a plot with axis descriptions.

Describe how to customize the color and line style of a plot line in matplotlib.

Remember the short string codes like 'r--' for red dashed lines.

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)