Legend placement and styling in Matplotlib - Mini Project: Build & Apply

Legend placement and styling

📖 Scenario: You are creating a simple line chart to compare sales data for two products over four quarters. You want to add a legend to clearly show which line belongs to which product. You will place the legend in a specific location and style it to make the chart easy to understand.

🎯 Goal: Build a line chart with two product sales lines, add a legend with labels, place the legend at the upper left corner, and style the legend with a shadow and a fancy box.

📋 What You'll Learn

Create two lists of sales data for Product A and Product B for four quarters.

Create a list of quarter labels for the x-axis.

Plot both sales lines with labels for the legend.

Add a legend placed at the upper left corner.

Style the legend with a shadow and a fancy box.

Display the plot.

💡 Why This Matters

🌍 Real World

Legends help viewers understand which data lines or points represent which categories in charts, making reports and presentations clearer.

💼 Career

Data scientists and analysts often need to create clear visualizations with legends to communicate insights effectively to stakeholders.

1

Create sales data and quarter labels

Create a list called quarters with values ["Q1", "Q2", "Q3", "Q4"]. Create two lists called sales_product_a with values [150, 200, 250, 300] and sales_product_b with values [180, 210, 240, 310].

Matplotlib

# Create the lists quarters, sales_product_a, and sales_product_b
# Your code here

Hint

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

2

Plot sales lines with labels

Import matplotlib.pyplot as plt. Use plt.plot() to plot sales_product_a and sales_product_b against quarters. Add labels "Product A" and "Product B" to each line for the legend.

Matplotlib

import matplotlib.pyplot as plt

# Plot sales_product_a with label 'Product A'
# Plot sales_product_b with label 'Product B'
# Your code here

Hint

Use plt.plot(x, y, label="...") to add labels for the legend.

3

Add and style the legend

Add a legend to the plot using plt.legend(). Place the legend at the upper left corner by setting loc='upper left'. Style the legend with a shadow by setting shadow=True and a fancy box by setting fancybox=True.

Matplotlib

plt.legend(loc='upper left', shadow=True, fancybox=True)
# Your code here

Hint

Use plt.legend() with the parameters loc, shadow, and fancybox.

4

Display the plot

Use plt.show() to display the plot with the legend.

Matplotlib

plt.show()
# Your code here

Hint

Call plt.show() to see the chart.

Practice

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

easy

A. The type of plot line used

B. The color of the legend text

C. The size of the plot markers

D. The position of the legend on the plot

Solution

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

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

2. Step 2: Differentiate from other styling options

   Other options control color, size, or line style, but loc only controls position.

3. Final Answer:

   The position of the legend on the plot -> Option D

4. Quick Check:

   Legend position = loc [OK]

Hint: Remember: loc means location of legend [OK]

Common Mistakes:

• Confusing loc with color or font size
• Thinking loc changes plot markers
• Assuming loc controls line style

2. Which of the following is the correct syntax to place a legend at the lower left corner in matplotlib?

easy

A. plt.legend(loc='lower left')

B. plt.legend(location='lower left')

C. plt.legend(loc='lower_left')

D. plt.legend(position='lower left')

Solution

1. Step 1: Identify the correct parameter name

   The correct parameter to set legend position is loc, not location or position.

2. Step 2: Use the correct string value

   The valid string is 'lower left' with a space, not 'lower_left' with underscore.

3. Final Answer:

   plt.legend(loc='lower left') -> Option A

4. Quick Check:

   Correct syntax uses loc='lower left' [OK]

Hint: Use loc='lower left' exactly, no underscores [OK]

Common Mistakes:

• Using 'location' instead of 'loc'
• Using underscores instead of spaces
• Using 'position' parameter which doesn't exist

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

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], label='Line A')
plt.plot([3, 2, 1], label='Line B')
plt.legend(loc='upper center', shadow=True, fontsize='small')
plt.show()

medium

A. Code will raise an error due to invalid fontsize

B. Legend appears at the top center with a shadow and small font

C. Legend appears inside the plot area without shadow

D. Legend appears at the bottom right with default font size

Solution

1. Step 1: Analyze legend parameters

   The loc='upper center' places the legend at the top center. shadow=True adds a shadow behind the legend box. fontsize='small' sets the font size smaller than default.

2. Step 2: Confirm no errors in parameters

   All parameters are valid and supported by matplotlib.

3. Final Answer:

   Legend appears at the top center with a shadow and small font -> Option B

4. Quick Check:

   loc + shadow + fontsize = top center, shadow, small font [OK]

Hint: Check loc for position, shadow=True adds shadow, fontsize sets text size [OK]

Common Mistakes:

• Assuming fontsize='small' causes error
• Confusing loc='upper center' with bottom right
• Ignoring shadow parameter effect

4. Identify the error in this code snippet for legend placement:

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], label='A')
plt.plot([3, 2, 1], label='B')
plt.legend(loc='bottomright')
plt.show()

medium

A. plt.legend() must be called before plt.plot()

B. Missing label parameter in plot calls

C. The loc value 'bottomright' is invalid; should be 'lower right'

D. plt.show() is missing parentheses

Solution

1. Step 1: Check the loc parameter value

   The valid location string is 'lower right', not 'bottomright'. 'bottomright' is not recognized by matplotlib.

2. Step 2: Verify other parts of the code

   Labels are correctly set, plt.legend() can be called after plotting, and plt.show() has parentheses.

3. Final Answer:

   The loc value 'bottomright' is invalid; should be 'lower right' -> Option C

4. Quick Check:

   Valid loc strings = 'lower right' not 'bottomright' [OK]

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

Common Mistakes:

• Using invalid loc strings
• Thinking plt.legend() must come before plt.plot()
• Forgetting parentheses in plt.show()

5. You want to place a legend outside the plot on the right side with a title 'Legend', font size 12, and a shadow. Which code snippet achieves this correctly?

hard

A. plt.legend(loc='center right', bbox_to_anchor=(1, 0.5), title='Legend', fontsize='12pt', shadow=True)

B. plt.legend(loc='right side', bbox_to_anchor=(1, 0.5), title='Legend', fontsize=12, shadow=True)

C. plt.legend(loc='center left', bbox_to_anchor=(1, 0.5), title='Legend', fontsize=12, shadow=True)

D. plt.legend(loc='outside right', bbox_to_anchor=(1, 0.5), title='Legend', fontsize=12, shadow=True)

Solution

1. Step 1: Understand placing legend outside plot

   Using loc='center right' with bbox_to_anchor=(1, 0.5) places the legend outside on the right side centered vertically.

2. Step 2: Check styling parameters

   Title 'Legend', font size 12 (string with units '12pt'), and shadow=True are correctly set. loc='right side' and loc='outside right' are invalid.

3. Final Answer:

   plt.legend(loc='center right', bbox_to_anchor=(1, 0.5), title='Legend', fontsize='12pt', shadow=True) -> Option A

4. Quick Check:

   loc + bbox_to_anchor + title + fontsize + shadow = correct legend outside right [OK]

Hint: Use loc='center right' with bbox_to_anchor=(1, 0.5) for outside right legend [OK]

Common Mistakes:

• Using invalid loc like 'right side' or 'outside right'
• Passing fontsize as integer without units
• Forgetting bbox_to_anchor for outside placement