Scatter Plots

When to Use Scatter Plots?

Scatter plots show relationships between two numbers:

Height vs Weight
Age vs Salary
Study hours vs Test score

Each dot is one data point.

Basic Scatter Plot

code.py

import matplotlib.pyplot as plt

age = [25, 30, 35, 40, 45, 50]
salary = [40000, 50000, 60000, 65000, 70000, 80000]

fig, ax = plt.subplots()
ax.scatter(age, salary)
ax.set_xlabel('Age')
ax.set_ylabel('Salary')
ax.set_title('Age vs Salary')
plt.show()

Change Marker Size

code.py

ax.scatter(age, salary, s=100)  # s = size

Change Color

code.py

ax.scatter(age, salary, color='red')

Color by Value

code.py

# Each point has different color based on value
sizes = [10, 20, 30, 40, 50, 60]

fig, ax = plt.subplots()
scatter = ax.scatter(age, salary, c=sizes, cmap='viridis')
plt.colorbar(scatter)  # Add color legend
plt.show()

Size by Value

code.py

# Bigger points for higher values
experience = [2, 5, 8, 12, 15, 20]

ax.scatter(age, salary, s=[x*20 for x in experience])

Multiple Groups

code.py

import matplotlib.pyplot as plt

# Group 1: Engineers
eng_age = [25, 30, 35, 40]
eng_salary = [60000, 70000, 80000, 90000]

# Group 2: Managers
mgr_age = [30, 35, 40, 45]
mgr_salary = [70000, 85000, 100000, 120000]

fig, ax = plt.subplots()
ax.scatter(eng_age, eng_salary, label='Engineers', color='blue')
ax.scatter(mgr_age, mgr_salary, label='Managers', color='red')

ax.set_xlabel('Age')
ax.set_ylabel('Salary')
ax.legend()
plt.show()

Add Transparency

code.py

ax.scatter(x, y, alpha=0.5)  # 0.5 = 50% transparent

Helpful when points overlap.

Change Marker Shape

code.py

ax.scatter(x, y, marker='s')  # squares
ax.scatter(x, y, marker='^')  # triangles
ax.scatter(x, y, marker='x')  # x marks

Complete Example

code.py

import matplotlib.pyplot as plt

# Student data
hours_studied = [1, 2, 3, 4, 5, 6, 7, 8]
test_scores = [50, 55, 65, 70, 75, 85, 88, 95]

fig, ax = plt.subplots(figsize=(8, 6))

ax.scatter(hours_studied, test_scores,
           s=100,
           c=test_scores,
           cmap='RdYlGn',
           alpha=0.7)

ax.set_xlabel('Hours Studied')
ax.set_ylabel('Test Score')
ax.set_title('Study Time vs Test Score')
ax.grid(True, alpha=0.3)

plt.show()

Key Points

scatter() creates scatter plots
s sets marker size
c sets color (can be array)
cmap sets color scheme
alpha sets transparency
marker sets shape
Great for showing correlations

What's Next?

Learn bar charts for comparing categories.