Cursor and event handling in Matplotlib

Introduction

Cursor and event handling lets you interact with plots. You can respond when someone moves the mouse or clicks on the graph.

You want to show data values when the mouse moves over a plot.

You want to let users click on points to get more info.

You want to change the plot based on mouse actions.

You want to create interactive visualizations for presentations.

You want to track mouse position on a graph for analysis.

Syntax

Matplotlib

fig, ax = plt.subplots()

# Define an event handler function
def on_move(event):
    if event.inaxes:
        print(f"Mouse at x={event.xdata:.2f}, y={event.ydata:.2f}")

# Connect the event to the handler
cid = fig.canvas.mpl_connect('motion_notify_event', on_move)

plt.show()

Use mpl_connect to link events like mouse move or click to your function.

The event object gives info like mouse position inside the plot.

Examples

This example prints the mouse coordinates as you move inside the plot area.

Matplotlib

fig, ax = plt.subplots()

# Print mouse position when moved inside plot

def on_move(event):
    if event.inaxes:
        print(f"Mouse at x={event.xdata:.1f}, y={event.ydata:.1f}")

fig.canvas.mpl_connect('motion_notify_event', on_move)
plt.show()

This example prints the coordinates where you click inside the plot.

Matplotlib

fig, ax = plt.subplots()

# Print message when mouse clicked

def on_click(event):
    if event.inaxes:
        print(f"Clicked at x={event.xdata:.2f}, y={event.ydata:.2f}")

fig.canvas.mpl_connect('button_press_event', on_click)
plt.show()

This example shows a red crosshair cursor that moves with the mouse on the plot.

Matplotlib

fig, ax = plt.subplots()

# Show a crosshair cursor following the mouse

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

plt.show()

Sample Program

This program plots points and shows a green crosshair cursor. When you click inside the plot, it prints the clicked coordinates.

Matplotlib

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

fig, ax = plt.subplots()
ax.plot([1, 2, 3, 4], [10, 20, 25, 30], marker='o')

# Create a cursor that follows the mouse
cursor = Cursor(ax, useblit=True, color='green', linewidth=1)

# Define event handler for mouse clicks
def on_click(event):
    if event.inaxes == ax:
        print(f"Clicked at x={event.xdata:.2f}, y={event.ydata:.2f}")

# Connect the click event to the handler
fig.canvas.mpl_connect('button_press_event', on_click)

plt.title('Click on points to see coordinates')
plt.show()

Important Notes

Event handlers must check if the event happened inside the plot area using event.inaxes.

The Cursor widget helps show a crosshair cursor easily.

Use mpl_connect to link many event types like mouse move, click, key press.

Summary

Cursor and event handling make plots interactive and responsive to mouse actions.

Use mpl_connect to connect mouse events to your functions.

The Cursor widget shows a crosshair that follows the mouse.

Practice

1. What is the main purpose of using mpl_connect in matplotlib?

easy

A. To create a new figure window

B. To connect an event like mouse click to a custom function

C. To save the current plot as an image file

D. To change the color of the plot lines

Solution

1. Step 1: Understand what mpl_connect does

   mpl_connect links events such as mouse clicks or key presses to functions you define.

2. Step 2: Identify the correct purpose

   It does not save images, create figures, or change colors directly. It connects events to functions.

3. Final Answer:

   To connect an event like mouse click to a custom function -> Option B

4. Quick Check:

   Event connection = C [OK]

Hint: Remember: mpl_connect links events to your functions [OK]

Common Mistakes:

• Thinking mpl_connect saves or modifies plots directly
• Confusing mpl_connect with figure creation
• Assuming mpl_connect changes plot styles

2. Which of the following is the correct syntax to connect a mouse click event to a function named on_click using matplotlib?

easy

A. fig.connect_event('button_press', on_click)

B. fig.mpl_connect('click', on_click)

C. fig.canvas.mpl_connect('button_press_event', on_click)

D. fig.connect('mouse_click', on_click)

Solution

1. Step 1: Recall the correct event name for mouse clicks

   The correct event name in matplotlib for mouse button press is 'button_press_event'.

2. Step 2: Check the syntax of mpl_connect

   The method is called on the figure's canvas as fig.canvas.mpl_connect(event_name, function).

3. Final Answer:

   fig.canvas.mpl_connect('button_press_event', on_click) -> Option C

4. Quick Check:

   Correct event name and syntax = A [OK]

Hint: Use 'button_press_event' for mouse clicks with mpl_connect [OK]

Common Mistakes:

• Using wrong event names like 'click' or 'mouse_click'
• Using non-existent methods like connect_event
• Mixing up method names and event strings

3. Consider the code below:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()

def on_move(event):
    print(f"Mouse at: {event.xdata}, {event.ydata}")

cid = fig.canvas.mpl_connect('motion_notify_event', on_move)
plt.show()

What will happen when you move the mouse over the plot area?

medium

A. The coordinates of the mouse pointer inside the plot will be printed continuously

B. Nothing will happen because the event is not connected properly

C. The plot will close immediately

D. An error will occur because event.xdata is undefined

Solution

1. Step 1: Understand the event type 'motion_notify_event'

   This event triggers whenever the mouse moves over the figure canvas.

2. Step 2: Analyze the function on_move

   The function prints the mouse coordinates inside the plot area using event.xdata and event.ydata.

3. Final Answer:

   The coordinates of the mouse pointer inside the plot will be printed continuously -> Option A

4. Quick Check:

   Mouse move event prints coords = B [OK]

Hint: motion_notify_event tracks mouse movement over plot [OK]

Common Mistakes:

• Assuming no output because event is not connected
• Thinking event.xdata is always None or undefined
• Expecting plot to close on mouse move

4. The following code is intended to print the mouse button pressed, but it raises an error:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()

def on_click(event):
    print(f"Button pressed: {event.button}")

fig.canvas.mpl_connect('button_press_event', on_click())
plt.show()

What is the error and how to fix it?

medium

A. The function is called immediately; remove parentheses in mpl_connect

B. The event name is wrong; use 'mouse_press' instead

C. event.button does not exist; use event.key instead

D. mpl_connect should be called on ax, not fig

Solution

1. Step 1: Identify the error in function connection

   Using on_click() calls the function immediately instead of passing it as a reference.

2. Step 2: Correct the function reference in mpl_connect

   Remove parentheses to pass the function itself: fig.canvas.mpl_connect('button_press_event', on_click).

3. Final Answer:

   The function is called immediately; remove parentheses in mpl_connect -> Option A

4. Quick Check:

   Pass function, not call it = D [OK]

Hint: Pass function name without () to mpl_connect [OK]

Common Mistakes:

• Calling the function instead of passing it
• Using wrong event names
• Trying to access wrong event attributes

5. You want to create an interactive plot where clicking inside the plot area prints the nearest data point's coordinates from a scatter plot. Which approach correctly combines cursor event handling and data lookup?

hard

A. Use plt.scatter() with a parameter to automatically print nearest point on click

B. Use 'motion_notify_event' to print coordinates continuously without checking points

C. Connect 'key_press_event' to print data points when any key is pressed

D. Connect 'button_press_event' to a function that calculates distances from click to all points and prints nearest

Solution

1. Step 1: Identify the correct event for mouse clicks

   Use 'button_press_event' to detect mouse clicks inside the plot.

2. Step 2: Implement logic to find nearest data point

   Calculate distances from click position to all scatter points, then print the closest one.

3. Step 3: Verify other options

   'motion_notify_event' prints continuously, 'key_press_event' is unrelated, and plt.scatter has no auto-print feature.

4. Final Answer:

   Connect 'button_press_event' to a function that calculates distances from click to all points and prints nearest -> Option D

5. Quick Check:

   Click event + nearest point logic = A [OK]

Hint: Use click event plus distance check to find nearest point [OK]

Common Mistakes:

• Using motion event instead of click for selection
• Expecting built-in auto-print in scatter
• Confusing key press with mouse click events