Multiple legends in Matplotlib - Mini Project: Build & Apply

Creating Multiple Legends in Matplotlib

📖 Scenario: You are working on a sales report for two different products over a week. You want to show their sales trends and also highlight their average sales with different styles on the same plot.

🎯 Goal: Build a line plot with two products' daily sales and add two separate legends: one for the sales lines and one for the average sales lines.

📋 What You'll Learn

Create two lists with daily sales data for Product A and Product B.

Create two variables for average sales of Product A and Product B.

Plot daily sales lines for both products.

Plot average sales lines for both products with a different style.

Add two separate legends: one for daily sales lines and one for average sales lines.

💡 Why This Matters

🌍 Real World

Multiple legends help show different groups of information clearly on one chart, like sales trends and averages.

💼 Career

Data scientists and analysts often need to present complex data visually with clear legends to help decision makers understand multiple data aspects.

1

Create sales data lists

Create two lists called sales_product_a and sales_product_b with these exact daily sales values: [10, 12, 15, 14, 13, 16, 18] for Product A and [8, 9, 11, 10, 12, 13, 15] for Product B.

Matplotlib

# Create sales data lists for Product A and Product B
# Your code here

Hint

Use square brackets to create lists with the exact numbers given.

2

Calculate average sales

Create two variables called avg_a and avg_b that store the average sales of sales_product_a and sales_product_b respectively. Use the sum() and len() functions.

Matplotlib

sales_product_a = [10, 12, 15, 14, 13, 16, 18]
sales_product_b = [8, 9, 11, 10, 12, 13, 15]
# Calculate average sales for Product A and Product B
# Your code here

Hint

Use sum() to add all values and len() to count the number of days.

3

Plot sales and average lines

Import matplotlib.pyplot as plt. Plot sales_product_a and sales_product_b as lines with labels 'Product A Sales' and 'Product B Sales'. Then plot horizontal lines for averages avg_a and avg_b with dashed style and labels 'Product A Average' and 'Product B Average'.

Matplotlib

sales_product_a = [10, 12, 15, 14, 13, 16, 18]
sales_product_b = [8, 9, 11, 10, 12, 13, 15]
avg_a = sum(sales_product_a) / len(sales_product_a)
avg_b = sum(sales_product_b) / len(sales_product_b)
# Import matplotlib and plot sales and average lines
# Your code here

Hint

Use plt.plot() for lines. Use 'r--' and 'g--' for dashed red and green lines.

4

Add two separate legends and show plot

Create two legends: one for the sales lines using plt.legend() with handles line1 and line2 placed at the upper left, and another for the average lines with handles line3 and line4 placed at the lower right. Then display the plot with plt.show().

Matplotlib

import matplotlib.pyplot as plt

sales_product_a = [10, 12, 15, 14, 13, 16, 18]
sales_product_b = [8, 9, 11, 10, 12, 13, 15]
avg_a = sum(sales_product_a) / len(sales_product_a)
avg_b = sum(sales_product_b) / len(sales_product_b)

days = range(1, 8)

line1, = plt.plot(days, sales_product_a, label='Product A Sales')
line2, = plt.plot(days, sales_product_b, label='Product B Sales')
line3, = plt.plot(days, [avg_a]*7, 'r--', label='Product A Average')
line4, = plt.plot(days, [avg_b]*7, 'g--', label='Product B Average')
# Add two legends and show the plot
# Your code here

Hint

Use plt.legend(handles=[...], loc='...') to create each legend. Use plt.gca().add_artist() to add the first legend before adding the second.

Practice

1. What is the main reason to use multiple legends in a matplotlib plot?

easy

A. To increase the plot size automatically

B. To change the color of the plot lines

C. To label different groups or data sets separately on the same plot

D. To save the plot as an image file

Solution

1. Step 1: Understand the purpose of legends

   Legends help identify what each line or marker in a plot represents.

2. Step 2: Recognize why multiple legends are needed

   When a plot has multiple groups or categories, separate legends make it easier to understand each group distinctly.

3. Final Answer:

   To label different groups or data sets separately on the same plot -> Option C

4. Quick Check:

   Multiple legends = separate group labels [OK]

Hint: Multiple legends label different groups clearly [OK]

Common Mistakes:

• Thinking legends change plot colors
• Confusing legends with saving files
• Assuming legends resize plots

2. Which matplotlib method is used to keep the first legend visible when adding a second legend?

easy

A. ax.add_artist()

B. plt.show()

C. plt.legend()

D. ax.set_title()

Solution

1. Step 1: Identify how legends are added

   Legends are usually added with plt.legend() or ax.legend().

2. Step 2: Understand how to keep multiple legends

   To keep the first legend visible after adding a second, you must add the first legend as an artist using ax.add_artist().

3. Final Answer:

   ax.add_artist() -> Option A

4. Quick Check:

   Keep first legend visible = add_artist() [OK]

Hint: Use add_artist() to keep first legend visible [OK]

Common Mistakes:

• Using plt.legend() twice without add_artist()
• Calling plt.show() to fix legends
• Trying to set title to fix legends

3. What will the following code display?

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
line1, = ax.plot([1, 2, 3], label='Line 1')
line2, = ax.plot([3, 2, 1], label='Line 2')
leg1 = ax.legend(handles=[line1], loc='upper left')
ax.add_artist(leg1)
ax.legend(handles=[line2], loc='upper right')
plt.show()

medium

A. Two legends: 'Line 1' at upper left and 'Line 2' at upper right

B. Only one legend showing 'Line 2' at upper right

C. Only one legend showing 'Line 1' at upper left

D. No legends displayed

Solution

1. Step 1: Analyze legend creation

   The first legend is created for 'Line 1' and added as an artist to keep it visible.

2. Step 2: Analyze second legend addition

   The second legend is created for 'Line 2' and placed at upper right. Both legends appear separately.

3. Final Answer:

   Two legends: 'Line 1' at upper left and 'Line 2' at upper right -> Option A

4. Quick Check:

   add_artist() + second legend = two legends [OK]

Hint: add_artist() keeps first legend when adding second [OK]

Common Mistakes:

• Expecting only one legend to show
• Forgetting to add_artist() for first legend
• Placing both legends at same location

4. Identify the error in this code that tries to add two legends:

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
line1, = ax.plot([1, 2, 3], label='A')
line2, = ax.plot([3, 2, 1], label='B')
ax.legend(handles=[line1], loc='upper left')
ax.legend(handles=[line2], loc='upper right')
plt.show()

medium

A. Syntax error due to missing parentheses

B. Second legend call overwrites the first, so only one legend shows

C. Plot lines are not labeled correctly

D. The plot will not display because plt.show() is missing

Solution

1. Step 1: Understand legend behavior

   Calling ax.legend() twice replaces the previous legend each time.

2. Step 2: Identify why only one legend appears

   Without using ax.add_artist(), the first legend is removed when the second is added.

3. Final Answer:

   Second legend call overwrites the first, so only one legend shows -> Option B

4. Quick Check:

   Multiple legend calls overwrite without add_artist() [OK]

Hint: Use add_artist() to keep multiple legends visible [OK]

Common Mistakes:

• Assuming multiple legend() calls stack automatically
• Thinking missing plt.show() causes no plot
• Believing labels cause syntax errors

5. You want to plot three data groups with separate legends placed at 'upper left', 'upper right', and 'lower left' without overlap. Which approach correctly achieves this?

hard

A. Plot all lines without legends and add a single legend manually

B. Call ax.legend() three times without add_artist(), changing loc each time

C. Use plt.legend() once with all labels combined

D. Create first legend, add it with ax.add_artist(), then create second and third legends with different loc values

Solution

1. Step 1: Understand legend placement

   Legends placed at different loc positions avoid overlap visually.

2. Step 2: Use add_artist() to keep multiple legends

   Adding the first legend as an artist keeps it visible when adding others.

3. Step 3: Create all three legends with distinct locations

   Creating legends separately with loc set to 'upper left', 'upper right', and 'lower left' places them without overlap.

4. Final Answer:

   Create first legend, add it with ax.add_artist(), then create second and third legends with different loc values -> Option D

5. Quick Check:

   add_artist() + distinct loc = multiple legends no overlap [OK]

Hint: Add first legend as artist, set loc differently for others [OK]

Common Mistakes:

• Calling legend() multiple times without add_artist()
• Placing all legends at same location causing overlap
• Using single legend for all groups losing clarity