Text placement with ax.text in Matplotlib - Step-by-Step Execution

Concept Flow - Text placement with ax.text

Create Figure and Axes

↓

Call ax.text(x, y, text)

↓

Calculate text position on plot

↓

Render text at (x, y)

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Display plot with text

The flow shows how matplotlib places text on a plot by specifying coordinates and the text string, then rendering it at that position.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot([1, 2, 3], [4, 5, 6])
ax.text(2, 5, 'Hello')
plt.show()

This code plots points and places the text 'Hello' at coordinates (2, 5) on the plot.

Execution Table

Step	Action	Input/Parameters	Result/Output
1	Create figure and axes	fig, ax = plt.subplots()	Figure and Axes objects created
2	Plot data	ax.plot([1,2,3], [4,5,6])	Line plotted through points (1,4), (2,5), (3,6)
3	Add text	ax.text(2, 5, 'Hello')	Text 'Hello' placed at x=2, y=5 on plot
4	Show plot	plt.show()	Plot window opens displaying line and text
5	Exit	Plot displayed	Execution ends

💡 Plot displayed and program ends

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
fig	None	Figure object	Figure object	Figure object	Figure object
ax	None	Axes object	Axes with line plotted	Axes with line and text	Axes with line and text

Key Moments - 2 Insights

Why does the text appear at (2, 5) and not at the data point (1, 4)?

Because ax.text places text exactly at the coordinates you give it, here (2, 5), independent of the plotted data points. See execution_table step 3.

What happens if the coordinates for ax.text are outside the plot limits?

The text will be placed outside the visible area and won't show on the plot. The coordinates must be within the axes limits to be visible.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the text 'Hello' added to the plot?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Check the 'Action' column in execution_table for when ax.text is called.

According to variable_tracker, what is the state of 'ax' after Step 3?

A Axes object with line and text

B Axes object with line plotted only

C Axes object with no plots or text

D None

💡 Hint

Look at the 'ax' row and 'After Step 3' column in variable_tracker.

If you change ax.text(2, 5, 'Hello') to ax.text(4, 7, 'Hi'), what will happen?

A Text 'Hi' will appear at (4,7) on the plot

B Text will not appear because (4,7) is outside plot limits

C Text 'Hello' will remain at (2,5)

D Plot will show an error

💡 Hint

ax.text places text exactly at given coordinates; changing them moves the text.

Concept Snapshot

ax.text(x, y, 'text') places text at coordinates (x, y) on the plot.
Coordinates are in data units by default.
Text appears exactly where specified.
Use after plotting data to annotate.
Coordinates outside axes limits won't show text.

Full Transcript

This visual execution shows how matplotlib's ax.text method places text on a plot. First, a figure and axes are created. Then data points are plotted as a line. Next, ax.text is called with coordinates (2, 5) and the string 'Hello', placing the text exactly at that point on the plot. Finally, plt.show() displays the plot with the line and the text. Variables 'fig' and 'ax' track the figure and axes states through these steps. Key points include understanding that text position is independent of data points and must be within plot limits to be visible. The quizzes test knowledge of when text is added, the state of axes, and effects of changing text coordinates.

Practice

1. What does the ax.text function do in matplotlib?

easy

A. It draws a line between two points.

B. It places text at specified x and y coordinates on the plot.

C. It creates a new figure window.

D. It changes the background color of the plot.

Solution

1. Step 1: Understand the purpose of ax.text

   The ax.text function is used to add text labels at specific positions on a plot using x and y coordinates.

2. Step 2: Compare with other options

   Other options describe different matplotlib functions: creating figures, drawing lines, or changing background color, which are not what ax.text does.

3. Final Answer:

   It places text at specified x and y coordinates on the plot. -> Option B

4. Quick Check:

   ax.text adds text labels = C [OK]

Hint: ax.text adds text at (x, y) on the plot [OK]

Common Mistakes:

• Confusing ax.text with plot or figure functions
• Thinking ax.text changes plot colors
• Assuming ax.text draws shapes

2. Which of the following is the correct syntax to place the text 'Hello' at coordinates (2, 3) using ax.text?

easy

A. ax.text(2, 3, 'Hello')

B. ax.text('Hello', 2, 3)

C. ax.text(x=3, y=2, text='Hello')

D. ax.text(3, 2, 'Hello')

Solution

1. Step 1: Recall ax.text parameter order

   The correct order is ax.text(x, y, text), so x=2, y=3, text='Hello' means ax.text(2, 3, 'Hello').

2. Step 2: Check other options

   ax.text('Hello', 2, 3) swaps text and coordinates, ax.text(x=3, y=2, text='Hello') uses wrong parameter names, ax.text(3, 2, 'Hello') swaps x and y values.

3. Final Answer:

   ax.text(2, 3, 'Hello') -> Option A

4. Quick Check:

   ax.text(x, y, text) = A [OK]

Hint: Remember ax.text(x, y, text) order [OK]

Common Mistakes:

• Swapping x and y coordinates
• Putting text before coordinates
• Using wrong parameter names

3. What will the following code display on the plot?

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot([1, 2, 3], [4, 5, 6])
ax.text(2, 5, 'Center', fontsize=12, color='red')
plt.show()

medium

A. An error because color must be uppercase.

B. A blue text 'Center' at point (2, 5) on the plot.

C. No text will appear because fontsize is missing.

D. A red text 'Center' at point (2, 5) on the plot.

Solution

1. Step 1: Analyze ax.text parameters

   The code places text 'Center' at (2, 5) with font size 12 and color red, which is valid syntax.

2. Step 2: Understand matplotlib behavior

   Text appears on the plot at the given coordinates with specified style. Color names are case-insensitive, so 'red' works fine.

3. Final Answer:

   A red text 'Center' at point (2, 5) on the plot. -> Option D

4. Quick Check:

   ax.text with color='red' shows red text = A [OK]

Hint: Color names in ax.text are case-insensitive [OK]

Common Mistakes:

• Thinking fontsize is mandatory to show text
• Assuming color must be uppercase
• Expecting text at wrong coordinates

4. Identify the error in this code snippet:

fig, ax = plt.subplots()
ax.text(1, 2, fontsize='large')
plt.show()

medium

A. plt.show() must come before ax.text.

B. ax.text requires x and y as strings, not numbers.

C. The text argument is missing.

D. fontsize must be an integer or float, not a string like 'large'.

Solution

1. Step 1: Check ax.text parameters

   Matplotlib's ax.text requires three positional arguments: x, y, and text (s). The code provides only x=1, y=2, then a keyword argument, missing the text.

2. Step 2: Verify other options

   Numbers for x/y are correct, fontsize='large' is valid, and plt.show() order is fine.

3. Final Answer:

   The text argument is missing. -> Option C

4. Quick Check:

   ax.text requires x, y, text = C [OK]

Hint: ax.text requires x, y, text positional args [OK]

Common Mistakes:

• Omitting the text argument
• Confusing parameter order
• Calling plt.show() too early

5. You want to add a label 'Max Point' exactly above the highest point in this plot:

import matplotlib.pyplot as plt
x = [1, 2, 3, 4]
y = [2, 5, 1, 7]
fig, ax = plt.subplots()
ax.plot(x, y, marker='o')
# Add text here
plt.show()

Which code correctly places the label 0.3 units above the highest point?

hard

A. ax.text(4, 7.3, 'Max Point', va='bottom', ha='center')

B. ax.text(4, 7.3, 'Max Point', ha='center')

C. ax.text(4, 7, 'Max Point', va='top')

D. ax.text(4, 7, 'Max Point', ha='left')

Solution

1. Step 1: Identify highest point coordinates

   The highest y value is 7 at x=4.

2. Step 2: Place text 0.3 units above with correct alignment

   To place text above, y=7.3 is correct. Use vertical alignment va='bottom' so text bottom aligns at 7.3, placing text above the point. Horizontal alignment ha='center' centers text horizontally at x=4.

3. Final Answer:

   ax.text(4, 7.3, 'Max Point', va='bottom', ha='center') -> Option A

4. Quick Check:

   Text above point uses y+offset and va='bottom' = B [OK]

Hint: Use va='bottom' to place text above y coordinate [OK]

Common Mistakes:

• Using va='top' places text below the coordinate
• Not adding offset to y coordinate
• Misaligning text horizontally