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.