Plotting multiple lines in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of plotting multiple lines with labels

What will be the output of this code snippet that plots two lines with labels and a legend?

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3]
y1 = [2, 3, 5]
y2 = [3, 5, 7]
plt.plot(x, y1, label='Line 1')
plt.plot(x, y2, label='Line 2')
plt.legend()
plt.show()

A A plot with two lines labeled 'Line 1' and 'Line 2' and a legend showing these labels.

B A plot with two lines but no legend or labels shown.

C A plot with two lines but the legend shows only 'Line 2'.

D A plot with one line only labeled 'Line 1'.

💡 Hint

Look at how labels and plt.legend() work together to show the legend.

The code plots two lines with labels and calls plt.legend(), so the legend shows both labels.

❓ data_output

intermediate

Number of lines plotted

How many lines will be drawn on the plot after running this code?

Matplotlib

import matplotlib.pyplot as plt
x = [0, 1, 2, 3]
for i in range(3):
    y = [j * (i + 1) for j in x]
    plt.plot(x, y)
plt.show()

A 4

B 3

C 1

D 0

💡 Hint

Count how many times plt.plot is called inside the loop.

The loop runs 3 times, each time plotting one line, so 3 lines total.

🔧 Debug

advanced

Identify the error in plotting multiple lines

What error will this code produce when trying to plot multiple lines?

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3]
y1 = [2, 3]
y2 = [3, 5, 7]
plt.plot(x, y1)
plt.plot(x, y2)
plt.show()

A ValueError: x and y must have same first dimension

B TypeError: 'list' object is not callable

C No error, plots two lines correctly

D NameError: name 'plt' is not defined

💡 Hint

Check if x and y lists have the same length for each plot call.

y1 has length 2 but x has length 3, causing a ValueError about dimension mismatch.

❓ visualization

advanced

Effect of linestyle on multiple lines

Which option describes the visual difference when plotting these two lines with different linestyles?

Matplotlib

import matplotlib.pyplot as plt
x = [0, 1, 2]
y1 = [1, 2, 3]
y2 = [3, 2, 1]
plt.plot(x, y1, linestyle='--')
plt.plot(x, y2, linestyle=':')
plt.show()

A Both lines are solid.

B One line is dotted and the other is solid.

C One line is dashed and the other is dotted.

D Both lines are dashed.

💡 Hint

Look up what '--' and ':' mean for linestyles in matplotlib.

'--' means dashed line, ':' means dotted line, so lines appear differently.

🚀 Application

expert

Plot multiple lines with different colors and labels

You want to plot three lines with x values [1,2,3], y values as multiples of x (1x, 2x, 3x), each with a different color and label. Which code produces the correct plot with legend?

A

import matplotlib.pyplot as plt
x = [1, 2, 3]
for i, color in enumerate(['red', 'green', 'blue']):
    y = [i * val for val in x]
    plt.plot(x, y, color=color)
plt.legend()
plt.show()

B

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [val for val in x]
plt.plot(x, y, color='red', label='Line 1')
plt.plot(x, [2*val for val in x], color='green')
plt.plot(x, [3*val for val in x], color='blue', label='Line 3')
plt.legend()
plt.show()

C

import matplotlib.pyplot as plt
x = [1, 2, 3]
for i in range(1, 4):
    y = [i * val for val in x]
    plt.plot(x, y, label='Line i')
plt.legend()
plt.show()

D

import matplotlib.pyplot as plt
x = [1, 2, 3]
for i, color in enumerate(['red', 'green', 'blue'], 1):
    y = [i * val for val in x]
    plt.plot(x, y, color=color, label=f'Line {i}')
plt.legend()
plt.show()

💡 Hint

Check how labels and colors are assigned and how enumerate is used.

Option D correctly assigns colors and labels using enumerate starting at 1, so labels match line multiples and legend shows all lines.

Practice

1. What is the main way to plot multiple lines on the same graph using matplotlib?

easy

A. Use plt.hist() multiple times

B. Use plt.scatter() for each line

C. Call plt.plot() multiple times with different data sets before plt.show()

D. Call plt.bar() for each line

Solution

1. Step 1: Understand plotting multiple lines

   In matplotlib, to plot multiple lines on the same graph, you call plt.plot() for each line's data before showing the plot.

2. Step 2: Identify correct plotting function

   plt.scatter(), plt.bar(), and plt.hist() are for scatter plots, bar charts, and histograms, not line plots.

3. Final Answer:

   Call plt.plot() multiple times with different data sets before plt.show() -> Option C

4. Quick Check:

   Multiple plt.plot() calls = multiple lines [OK]

Hint: Use multiple plt.plot() calls to add lines [OK]

Common Mistakes:

• Using plt.scatter() instead of plt.plot() for lines
• Calling plt.show() after each line instead of once
• Using bar or histogram functions for line plots

2. Which of the following code snippets correctly plots two lines on the same graph?

easy

A. plt.plot([1, 2, 3], [4, 5, 6])
plt.plot([1, 2, 3], [6, 5, 4])
plt.show()

B. plt.plot([1, 2, 3], [4, 5, 6])
plt.show()
plt.plot([1, 2, 3], [6, 5, 4])

C. plt.show()
plt.plot([1, 2, 3], [4, 5, 6])
plt.plot([1, 2, 3], [6, 5, 4])

D. plt.plot([1, 2, 3], [4, 5, 6])
plt.plot([1, 2, 3], [6, 5, 4])

Solution

1. Step 1: Check order of plotting and showing

   To plot multiple lines, call plt.plot() for each line, then call plt.show() once at the end.

2. Step 2: Identify correct snippet

   plt.plot([1, 2, 3], [4, 5, 6]) plt.plot([1, 2, 3], [6, 5, 4]) plt.show() calls plt.plot() twice then plt.show(), which is correct. plt.plot([1, 2, 3], [4, 5, 6]) plt.show() plt.plot([1, 2, 3], [6, 5, 4]) calls plt.show() too early, splitting the plots. plt.show() plt.plot([1, 2, 3], [4, 5, 6]) plt.plot([1, 2, 3], [6, 5, 4]) calls plt.show() before plotting, so nothing shows. plt.plot([1, 2, 3], [4, 5, 6]) plt.plot([1, 2, 3], [6, 5, 4]) misses plt.show() entirely.

3. Final Answer:

   Two plt.plot() calls followed by plt.show() -> Option A

4. Quick Check:

   Plot lines first, show once after [OK]

Hint: Call plt.show() only after all plt.plot() calls [OK]

Common Mistakes:

• Calling plt.show() before all plots
• Forgetting plt.show() to display plot
• Plotting after plt.show() call

3. What will be the output of this code?

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

medium

A. A graph with two lines but no legend

B. A graph with two lines labeled 'Line 1' and 'Line 2'

C. A graph with one line labeled 'Line 2' only

D. No graph displayed due to error

Solution

1. Step 1: Analyze plotting commands

   The code plots two lines with plt.plot() and assigns labels 'Line 1' and 'Line 2'.

2. Step 2: Understand legend usage

   Calling plt.legend() displays the legend with the labels for both lines.

3. Final Answer:

   A graph with two lines labeled 'Line 1' and 'Line 2' -> Option B

4. Quick Check:

   Labels + plt.legend() = labeled lines [OK]

Hint: Use plt.legend() to show labels for all lines [OK]

Common Mistakes:

• Forgetting plt.legend() so labels don't show
• Assigning labels incorrectly
• Plotting only one line

4. Identify the error in this code that tries to plot two lines:

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

medium

A. Missing parentheses after plt.legend() call

B. x and y lists have different lengths

C. plt.show() is called before plotting

D. Labels are not assigned to lines

Solution

1. Step 1: Check function calls

   The code calls plt.legend without parentheses, so the legend function is not executed.

2. Step 2: Understand effect of missing parentheses

   Without calling plt.legend(), the legend does not appear, so labels won't show.

3. Final Answer:

   Missing parentheses after plt.legend() call -> Option A

4. Quick Check:

   Call plt.legend() with () to show legend [OK]

Hint: Always add () after plt.legend to display legend [OK]

Common Mistakes:

• Forgetting () after plt.legend
• Assuming legend shows without calling function
• Mixing up order of plt.show()

5. You want to plot three lines with different colors and styles on the same graph and add a legend. Which code snippet correctly achieves this?

hard

A. plt.plot(x, y1, 'r-', label='Red solid')
plt.plot(x, y2, 'g--')
plt.plot(x, y3, 'b:')
plt.legend()
plt.show()

B. plt.plot(x, y1, color='red', linestyle='-', label='Red solid')
plt.plot(x, y2, color='green', linestyle='--', label='Green dashed')
plt.plot(x, y3, color='blue', linestyle=':', label='Blue dotted')
plt.legend()
plt.show()

C. plt.plot(x, y1, 'red', 'solid', label='Red solid')
plt.plot(x, y2, 'green', 'dashed', label='Green dashed')
plt.plot(x, y3, 'blue', 'dotted', label='Blue dotted')
plt.legend()
plt.show()

D. plt.plot(x, y1, 'r-', label='Red solid')
plt.plot(x, y2, 'g--', label='Green dashed')
plt.plot(x, y3, 'b:', label='Blue dotted')
plt.legend()
plt.show()

Solution

1. Step 1: Understand color and style shorthand

   In matplotlib, shorthand like 'r-' means red solid line, 'g--' green dashed, 'b:' blue dotted.

2. Step 2: Check label usage and legend

   plt.plot(x, y1, 'r-', label='Red solid') plt.plot(x, y2, 'g--', label='Green dashed') plt.plot(x, y3, 'b:', label='Blue dotted') plt.legend() plt.show() assigns labels to all lines and calls plt.legend() to show them. plt.plot(x, y1, color='red', linestyle='solid', label='Red solid') plt.plot(x, y2, color='green', linestyle='dashed', label='Green dashed') plt.plot(x, y3, color='blue', linestyle='dotted', label='Blue dotted') plt.legend() plt.show() uses invalid linestyle names ('solid', 'dashed', 'dotted' are not valid; correct are '-', '--', ':'). plt.plot(x, y1, 'red', 'solid', label='Red solid') plt.plot(x, y2, 'green', 'dashed', label='Green dashed') plt.plot(x, y3, 'blue', 'dotted', label='Blue dotted') plt.legend() plt.show() uses positional strings incorrectly. plt.plot(x, y1, 'r-', label='Red solid') plt.plot(x, y2, 'g--') plt.plot(x, y3, 'b:') plt.legend() plt.show() misses labels for second and third lines.

3. Final Answer:

   Shorthand format strings like 'r-', 'g--', 'b:' for each plt.plot() -> Option D

4. Quick Check:

   Use shorthand 'color-style' and label each line [OK]

Hint: Use shorthand like 'r-', 'g--', 'b:' with labels [OK]

Common Mistakes:

• Using invalid linestyle names like 'solid'
• Forgetting labels on all lines
• Not calling plt.legend() to show labels