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Scatter Graphs (matplotlib)

🟨🐍 Technique Card: Scatter Graphs with matplotlib (practical)

🎯 What You’ll Learn

  • How to use matplotlib to make a scatter graph in Python.
  • How to plot x and y values.
  • How to spot correlation patterns.

🧰 You Will Need

import matplotlib.pyplot as plt

✏️ Example: Plotting Dummy Data

Here’s some simple data showing hours revised (x-axis) and test scores (y-axis).

import matplotlib.pyplot as plt

# Dummy data
hours_revised = [1, 2, 3, 4, 5, 6, 7]
test_scores = [52, 55, 60, 63, 67, 72, 75]

# Create the scatter plot
plt.scatter(hours_revised, test_scores)

# Add labels and title
plt.xlabel("Hours Revised")
plt.ylabel("Test Score (%)")
plt.title("Revision Time vs Test Score")

# Show the graph
plt.show()

👀 What You’ll See

A scatter graph where the points go upwards as the hours increase – this is a positive correlation.

🔍 Patterns to Look For

Pattern Type What It Looks Like What It Means
Positive Points rise together ↗️ More x = more y
Negative One goes up, the other down ↘️ More x = less y
No Correlation Points are random, no clear pattern ❓ No clear link between x and y

📉 Negative and No Correlation

What happens to the data points on the scatter graph when you start to plot the following data...

hours_tv_watched = [1, 2, 3, 4, 5, 6, 7]
test_scores = [95, 88, 81, 75, 68, 54, 41]

...and how about with this data...

hours_pet_sleeps = [1, 2, 3, 4, 5, 6, 7]
test_scores = [83, 45, 56, 52, 75, 38, 61]

💡 Practise

  • Try creating scatter graphs base on data you provide in your code - can you get positive correlation? How about negative correlation? And no correlation?
  • Use data from a dataset provided in our projects - would a scatter graph help you answer any questions you may have about it?
  • Collect some real data yourself and see if a scatter graph can help you understand it.

🏷️ Adding Labels to the Data Points

To label each data point on a scatter plot in Matplotlib, you can use the plt.text() function to add a label next to each point.

✏️ Example with Labels on Each Point

import matplotlib.pyplot as plt

# Dummy data
hours_revised = [1, 2, 3, 4, 5, 6, 7]
test_scores = [52, 55, 60, 63, 67, 72, 75]
student_names = ["Ali", "Ben", "Chloe", "Dani", "Eve", "Finn", "Grace"]

# Create the scatter plot
plt.scatter(hours_revised, test_scores)

# Add labels to each point
for i in range(len(student_names)):
    plt.text(hours_revised[i] + 0.1, test_scores[i], student_names[i], fontsize=9)

# Add axis labels and title
plt.xlabel("Hours Revised")
plt.ylabel("Test Score (%)")
plt.title("Revision Time vs Test Score")

# Show the graph
plt.show()

💡 Notes:

  • + 0.1 moves the label slightly to the right so it doesn’t sit right on top of the dot.
  • You can tweak this for better readability: move the label above, below, or next to the point by adjusting the x and y positions.

🔗 Linked Cards

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