Cursor and event handling in Matplotlib - Step-by-Step Execution

Concept Flow - Cursor and event handling

Start plot display

↓

User moves mouse

↓

Event triggered: mouse move

↓

Cursor position updated

↓

Display updated with cursor info

↓

User clicks or closes plot

↓

Event triggered: click or close

↓

Handle event or exit

The flow shows how matplotlib listens for mouse events, updates cursor info on move, and handles clicks or closing events.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
from matplotlib.widgets import Cursor

fig, ax = plt.subplots()

cursor = Cursor(ax, useblit=True, color='red', linewidth=1)

plt.show()

This code creates a plot with a red cursor line that follows the mouse pointer.

Execution Table

Step	Event	Cursor Position	Action	Output
1	Plot shown	None	Wait for mouse events	Plot window appears
2	Mouse move at (0.5, 0.5)	(0.5, 0.5)	Update cursor lines	Cursor lines drawn at x=0.5 and y=0.5
3	Mouse move at (0.7, 0.3)	(0.7, 0.3)	Update cursor lines	Cursor lines move to x=0.7 and y=0.3
4	Mouse click at (0.7, 0.3)	(0.7, 0.3)	Trigger click event handler	Print or handle click info (if defined)
5	Plot window closed	None	Trigger close event handler	Program ends
6	No more events	None	Exit	Plot closed, program stops

💡 Plot window closed or user exits, stopping event listening

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
cursor position	None	(0.5, 0.5)	(0.7, 0.3)	(0.7, 0.3)	None (plot closed)
event type	None	mouse move	mouse move	mouse click	close

Key Moments - 3 Insights

Why does the cursor position update only when the mouse moves?

Because the cursor update is tied to the mouse move event (see execution_table steps 2 and 3). Without mouse movement, no event triggers an update.

What happens if no event handler is defined for mouse clicks?

The click event occurs (step 4), but no action happens unless you define a handler. The cursor still shows position but no extra output.

Why does the program stop after closing the plot window?

Closing the plot triggers the close event (step 5), which ends the event loop and stops the program (step 6).

Visual Quiz - 3 Questions

Look at the execution table, what is the cursor position at step 3?

A (0.7, 0.3)

B None

C (0.5, 0.5)

D (0.3, 0.7)

💡 Hint

Check the 'Cursor Position' column at step 3 in the execution_table.

At which step does the program stop listening for events?

A Step 4

B Step 5

C Step 6

D Step 2

💡 Hint

Look at the 'exit_note' and the last rows in the execution_table.

If the mouse never moves, what will the cursor position be?

A Always (0,0)

B None

C Last clicked position

D Random position

💡 Hint

Refer to variable_tracker for 'cursor position' start value and step 2.

Concept Snapshot

matplotlib cursor and event handling:
- Use Cursor(ax) to show crosshair following mouse
- Connect event handlers with fig.canvas.mpl_connect(event, handler)
- Events: 'motion_notify_event' for mouse move, 'button_press_event' for clicks
- Cursor updates only on mouse move events
- Closing plot stops event listening and ends program

Full Transcript

This visual execution shows how matplotlib handles cursor and mouse events. When the plot window opens, it waits for mouse events. Moving the mouse triggers 'motion_notify_event', updating the cursor position and redrawing crosshair lines at the mouse location. Clicking triggers 'button_press_event', which can be handled to perform actions. Closing the plot triggers a close event that stops the program. Variables like cursor position update only on mouse moves. This step-by-step trace helps beginners see how event-driven cursor updates work in matplotlib.