Legend placement options in Matplotlib - Mini Project: Build & Apply

Legend placement options

📖 Scenario: You are creating a simple line chart to compare sales of two products over 5 months. You want to add a legend to explain which line is which. The legend can be placed in different positions on the chart.

🎯 Goal: Learn how to add a legend to a matplotlib plot and place it in different positions using the loc parameter.

📋 What You'll Learn

Create a line plot with two lines representing sales data for Product A and Product B.

Add a legend with labels for each product.

Set the legend position using the loc parameter with a string value.

💡 Why This Matters

🌍 Real World

Legends help explain different data series in charts, making them easier to understand in reports and presentations.

💼 Career

Data scientists and analysts often create visualizations with legends to clearly communicate insights to stakeholders.

1

Create sales data and plot lines

Create two lists called months and sales_a with values [1, 2, 3, 4, 5] and [10, 15, 13, 17, 20] respectively. Also create a list called sales_b with values [12, 14, 16, 18, 22]. Then import matplotlib.pyplot as plt and plot sales_a and sales_b against months using plt.plot(). Use labels 'Product A' and 'Product B' for the lines.

Matplotlib

# Create months, sales_a, sales_b lists
# Import matplotlib.pyplot as plt
# Plot sales_a and sales_b against months with labels

Hint

Use plt.plot(x_values, y_values, label='name') to plot lines with labels.

2

Add a legend with default placement

Add a legend to the plot by calling plt.legend() without any arguments.

Matplotlib

months = [1, 2, 3, 4, 5]
sales_a = [10, 15, 13, 17, 20]
sales_b = [12, 14, 16, 18, 22]
import matplotlib.pyplot as plt
plt.plot(months, sales_a, label='Product A')
plt.plot(months, sales_b, label='Product B')
# Add a legend with default placement

Hint

Call plt.legend() to add the legend with default position.

3

Change legend position to upper left

Modify the legend call to place the legend in the upper left corner by passing loc='upper left' to plt.legend().

Matplotlib

months = [1, 2, 3, 4, 5]
sales_a = [10, 15, 13, 17, 20]
sales_b = [12, 14, 16, 18, 22]
import matplotlib.pyplot as plt
plt.plot(months, sales_a, label='Product A')
plt.plot(months, sales_b, label='Product B')
# Change legend position to upper left

Hint

Use plt.legend(loc='upper left') to move the legend.

4

Display the plot

Add a line to display the plot by calling plt.show().

Matplotlib

months = [1, 2, 3, 4, 5]
sales_a = [10, 15, 13, 17, 20]
sales_b = [12, 14, 16, 18, 22]
import matplotlib.pyplot as plt
plt.plot(months, sales_a, label='Product A')
plt.plot(months, sales_b, label='Product B')
plt.legend(loc='upper left')
# Display the plot

Hint

Use plt.show() to display the plot window.

Practice

1. What does the loc parameter control in a matplotlib legend?

easy

A. The type of plot displayed

B. The color of the legend box

C. The font size of the legend text

D. The position of the legend inside or outside the plot area

Solution

1. Step 1: Understand the role of loc in legend placement

   The loc parameter specifies where the legend appears relative to the plot, such as 'upper right' or 'lower left'.

2. Step 2: Differentiate from other legend properties

   Other properties like color or font size are controlled by different parameters, not loc.

3. Final Answer:

   The position of the legend inside or outside the plot area -> Option D

4. Quick Check:

   Legend position = loc [OK]

Hint: Remember: loc sets legend position, not style [OK]

Common Mistakes:

• Confusing loc with color or font size
• Thinking loc controls plot type
• Assuming loc changes data points

2. Which of the following is the correct syntax to place a legend at the lower center outside the plot using loc and bbox_to_anchor?

easy

A. plt.legend(loc='lower center', bbox_to_anchor=(0.5, -0.1))

B. plt.legend(loc='center lower', bbox_to_anchor=(0.5, -0.1))

C. plt.legend(location='lower center', bbox_to_anchor=(0.5, -0.1))

D. plt.legend(loc='lower center', bbox_to_anchor=[0.5, -0.1])

Solution

1. Step 1: Check correct parameter names and values

   The parameter for legend position is loc, and the value 'lower center' is valid. The bbox_to_anchor expects a tuple for coordinates.

2. Step 2: Identify syntax errors in options

   plt.legend(loc='center lower', bbox_to_anchor=(0.5, -0.1)) uses 'center lower' which is invalid. plt.legend(location='lower center', bbox_to_anchor=(0.5, -0.1)) uses 'location' instead of 'loc'. plt.legend(loc='lower center', bbox_to_anchor=[0.5, -0.1]) uses a list instead of a tuple for bbox_to_anchor.

3. Final Answer:

   plt.legend(loc='lower center', bbox_to_anchor=(0.5, -0.1)) -> Option A

4. Quick Check:

   Correct loc and tuple bbox_to_anchor = plt.legend(loc='lower center', bbox_to_anchor=(0.5, -0.1)) [OK]

Hint: Use loc='position' and bbox_to_anchor=(x, y) tuple [OK]

Common Mistakes:

• Using wrong parameter name 'location' instead of 'loc'
• Swapping order in loc value like 'center lower'
• Passing list instead of tuple for bbox_to_anchor

3. What will be the effect of this code snippet?

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], label='Line')
plt.legend(loc='upper left', bbox_to_anchor=(1, 1))
plt.show()

medium

A. Legend appears inside the plot at upper left corner

B. Legend appears outside the plot at the upper left corner

C. Legend appears outside the plot at the top-right corner

D. Code raises an error due to invalid bbox_to_anchor

Solution

1. Step 1: Understand loc and bbox_to_anchor interaction

   The loc='upper left' sets the legend's anchor point to its upper left corner, and bbox_to_anchor=(1,1) places that anchor point at the top-right corner of the axes.

2. Step 2: Determine legend position

   With the legend's upper left anchored at the plot's top-right corner (1,1), the legend extends to the right, appearing outside the plot at the top-right corner.

3. Final Answer:

   Legend appears outside the plot at the top-right corner -> Option C

4. Quick Check:

   loc='upper left' + bbox_to_anchor=(1,1) = outside top-right [OK]

Hint: bbox_to_anchor moves legend outside; loc sets anchor point [OK]

Common Mistakes:

• Assuming legend stays inside plot
• Confusing loc and bbox_to_anchor roles
• Expecting an error from bbox_to_anchor values

4. Identify the error in this code that tries to place the legend outside the plot at the bottom right:

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], label='Data')
plt.legend(loc='bottom right', bbox_to_anchor=(1, 0))
plt.show()

medium

A. Label parameter is incorrect

B. Invalid loc value 'bottom right'

C. Missing plt.show() call

D. bbox_to_anchor must be a list, not a tuple

Solution

1. Step 1: Check valid loc values

   Matplotlib does not recognize 'bottom right' as a valid loc string. The correct value is 'lower right'.

2. Step 2: Verify other parameters

   bbox_to_anchor accepts tuples, plt.show() is present, and label syntax is correct.

3. Final Answer:

   Invalid loc value 'bottom right' -> Option B

4. Quick Check:

   loc must be valid string like 'lower right' [OK]

Hint: Use exact loc strings like 'lower right', not 'bottom right' [OK]

Common Mistakes:

• Using incorrect loc names
• Thinking bbox_to_anchor needs list
• Forgetting plt.show()

5. You want to place a legend outside the plot on the right side, vertically centered. Which combination of loc and bbox_to_anchor achieves this?

hard

A. loc='center left', bbox_to_anchor=(1, 0.5)

B. loc='center right', bbox_to_anchor=(1, 0.5)

C. loc='center left', bbox_to_anchor=(0, 0.5)

D. loc='upper right', bbox_to_anchor=(1, 0.5)

Solution

1. Step 1: Understand legend anchor and position

   To place the legend outside on the right, the anchor point should be on the left side of the legend box, so it aligns just outside the plot.

2. Step 2: Choose correct loc and bbox_to_anchor

   loc='center left' anchors the legend's left center, and bbox_to_anchor=(1, 0.5) places it just outside the plot on the right, vertically centered.

3. Final Answer:

   loc='center left', bbox_to_anchor=(1, 0.5) -> Option A

4. Quick Check:

   Anchor left + bbox at right center = legend outside right [OK]

Hint: Anchor legend left side to place it outside right [OK]

Common Mistakes:

• Using loc='center right' which anchors wrong side
• Setting bbox_to_anchor x=0 to place outside right
• Using upper right loc for vertical center