Why Does plt.show() Display My Plot?

[Rajeev Bagra]

 

Understanding How Matplotlib Handles Figures (Beginner to Intermediate Guide)

When learning Matplotlib, many beginners ask:

“If I used ax.boxplot() to create the plot, why does plt.show() display it?”
“What if there are multiple figures open?”
“How do I display only the latest plot?”

Today, let’s break this down clearly.

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📌 Step 1: What Is plt?

When we write:

import matplotlib.pyplot as plt

plt is simply an alias for the matplotlib.pyplot module.

The pyplot module acts like a figure manager — it keeps track of all figures created during a session.

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📊 Creating a Boxplot (Object-Oriented Style)

Example:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()

ax.boxplot(area_m2, vert=False)
ax.set_xlabel("Area [sq meters]")
ax.set_title("Distribution of Home Sizes")

plt.show()

Here’s what happens internally:

• plt.subplots() creates:
  • A Figure object (fig)
  • An Axes object (ax)
• ax.boxplot() draws on that figure
• plt.show() tells Matplotlib:

“Render all currently active figures.”

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🔎 Important Concept: Active Figures

Matplotlib keeps every created figure in memory until you close it.

So if you previously ran:

fig1, ax1 = plt.subplots()
ax1.plot([1, 2, 3])

And then later:

fig2, ax2 = plt.subplots()
ax2.plot([3, 2, 1])
plt.show()

👉 Both figures will display.

Because both are still active.

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⚠️ Why Does This Happen?

matplotlib.pyplot works as a state machine.

Internally, it maintains a list of open figures.
When plt.show() runs, it loops through that list and renders everything.

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✅ How to Display Only the Latest Plot

🔹 Method 1 (Best Practice in Scripts)

Reset the environment before plotting:

plt.close('all')

Then create your new figure:

fig, ax = plt.subplots()
ax.boxplot(area_m2, vert=False)

ax.set_xlabel("Area [sq meters]")
ax.set_title("Distribution of Home Sizes")

plt.show()

Now only one figure exists — so only one displays.

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🔹 Method 2 — Show Specific Figure

Since you stored the figure:

fig.show()

This displays only that figure.

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🔹 Method 3 — In Jupyter Notebooks

If the figure object is the last line:

fig

Jupyter automatically renders just that figure.

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🎨 Two Styles of Using Matplotlib

1️⃣ State-Based Style

plt.boxplot(area_m2)
plt.show()

2️⃣ Object-Oriented Style (Recommended)

fig, ax = plt.subplots()
ax.boxplot(area_m2)
plt.show()

Even in object-oriented style, plt.show() still works because pyplot manages the figures globally.

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🧠 Mental Model (Very Important)

Think of it like this:

• fig = Canvas
• ax = Drawing area
• ax.boxplot() = Drawing on the canvas
• plt.show() = Hanging all canvases on the wall

If multiple canvases exist → all get displayed.

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🎯 Key Takeaways

✔ plt is the alias for matplotlib.pyplot
✔ plt.show() displays all open figures
✔ Matplotlib internally tracks active figures
✔ Use plt.close('all') to avoid showing old plots
✔ Object-oriented style is more scalable and professional

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