Skip to content

Module 07 / Project 04 — Visualization

README · Module Index

Learn Your Way

Read Build Watch Test Review Visualize Try
— This project — — Flashcards — —

Focus

  • matplotlib basics: figure(), plot(), bar(), scatter()
  • Adding titles, axis labels, and legends
  • Creating multi-panel figures with subplots()
  • Saving figures to files with savefig()
  • Using the Agg backend for non-interactive environments

Why this project exists

Numbers in a table are hard to interpret. A chart makes patterns, outliers, and trends visible at a glance. This project teaches you how to create four common chart types with matplotlib — the standard Python plotting library. You will also learn how to combine multiple charts into a single figure and save the result to a file, which is how charts are shared in reports and presentations.

Run

cd projects/modules/07-data-analysis/04-visualization
python project.py

Expected output

=== Loading student data ===
Loaded 30 rows from data/students.csv

=== Chart 1: Bar chart — Average grade by subject ===
Created bar chart.

=== Chart 2: Line chart — Grades over student index ===
Created line chart.

=== Chart 3: Scatter plot — Age vs grade ===
Created scatter plot.

=== Chart 4: Combined 2x2 subplot ===
Saved combined figure to data/charts.png

Done. Open data/charts.png to see all four charts.

After running, open data/charts.png to see the four charts arranged in a 2x2 grid.

Alter it

  1. Change the bar chart colors. Try color="coral" or color=["#e74c3c", "#3498db", "#2ecc71"].
  2. Add a horizontal line to the line chart at the class average: ax.axhline(y=mean_grade, color="red", linestyle="--").
  3. Change the scatter plot to use different colors per subject. (Hint: loop through subjects and plot each one separately with a label.)
  4. Add a histogram of all grades as a fifth chart. Change the subplot grid to 2x3 or 3x2.

Break it

  1. Remove the matplotlib.use("Agg") line and run on a machine with no display. What error do you get?
  2. Try creating a subplot grid of 2x2 but only fill 3 panels. What does the empty panel look like?
  3. Pass a non-existent column name to the plot function. Read the error.

Fix it

  1. Always set the backend before importing pyplot: matplotlib.use("Agg") must come before import matplotlib.pyplot as plt.
  2. Hide unused subplot panels with ax.set_visible(False).
  3. Check that columns exist before plotting: if column_name in df.columns.

Explain it

  1. What is the difference between plt.plot() and ax.plot()? When would you use each?
  2. What does matplotlib.use("Agg") do? Why is it needed in scripts that save to files?
  3. What is a "figure" vs an "axes" in matplotlib? How do they relate?
  4. Why does savefig() need to be called before plt.show()?

Mastery check

You can move on when you can:

  • Create a bar chart, line chart, and scatter plot from a DataFrame.
  • Add titles, axis labels, and a legend to any chart.
  • Create a multi-panel figure with subplots().
  • Save a figure to a PNG file.
  • Explain the difference between figure, axes, and plot.

Next

Project 05 — Analysis Report