Cursor and event handling in Matplotlib - Mini Project: Build & Apply

Interactive Plot with Cursor and Event Handling

📖 Scenario: You are working with a simple line plot showing sales data over a week. You want to make the plot interactive so that when you move your mouse over the plot, a cursor line follows your mouse, and when you click on the plot, the exact data point coordinates are printed.

🎯 Goal: Create a matplotlib plot with a cursor that moves with the mouse and prints the coordinates of the clicked point.

📋 What You'll Learn

Create a line plot with given sales data.

Add a vertical cursor line that follows the mouse movement.

Print the x and y coordinates when the plot is clicked.

💡 Why This Matters

🌍 Real World

Interactive plots help analysts explore data visually and get exact values by moving the mouse or clicking points.

💼 Career

Data scientists and analysts often create interactive visualizations to communicate insights clearly and allow users to explore data.

1

Create the sales data and plot

Create a list called days with values [1, 2, 3, 4, 5, 6, 7] representing days of the week. Create a list called sales with values [10, 15, 7, 12, 20, 18, 25]. Then, import matplotlib.pyplot as plt and plot sales against days using plt.plot(days, sales). Finally, call plt.show() to display the plot.

Matplotlib

# Create days and sales lists
# Import matplotlib.pyplot as plt
# Plot sales vs days
# Show the plot
# Your code here

Hint

Remember to import matplotlib.pyplot as plt before plotting.

2

Add a vertical cursor line

After plotting, create a variable called cursor_line that stores a vertical line on the plot using plt.axvline at position x=0 with color 'gray' and linestyle '--'. This line will act as the cursor.

Matplotlib

days = [1, 2, 3, 4, 5, 6, 7]
sales = [10, 15, 7, 12, 20, 18, 25]
import matplotlib.pyplot as plt
plt.plot(days, sales)
# Create a vertical line at x=0 with gray dashed style and assign it to cursor_line
# Your code here
plt.show()

Hint

Use plt.axvline to add a vertical line at x=0.

3

Add mouse move event to update cursor

Define a function called on_mouse_move that takes an event parameter. Inside the function, check if event.xdata is not None. If so, update the cursor_line position by setting its xdata to [event.xdata, event.xdata] and then call plt.gcf().canvas.draw_idle() to refresh the plot. Connect this function to the figure's motion_notify_event using plt.gcf().canvas.mpl_connect.

Matplotlib

days = [1, 2, 3, 4, 5, 6, 7]
sales = [10, 15, 7, 12, 20, 18, 25]
import matplotlib.pyplot as plt
plt.plot(days, sales)
cursor_line = plt.axvline(x=0, color='gray', linestyle='--')
# Define on_mouse_move(event) to update cursor_line position
# Connect on_mouse_move to motion_notify_event
plt.show()

Hint

Use set_xdata on the cursor line and redraw the canvas.

4

Add click event to print coordinates

Define a function called on_click that takes an event parameter. Inside the function, check if event.xdata and event.ydata are not None. If so, print the message Clicked at x={event.xdata:.2f}, y={event.ydata:.2f} using an f-string. Connect this function to the figure's button_press_event using plt.gcf().canvas.mpl_connect. Finally, call plt.show() to display the interactive plot.

Matplotlib

days = [1, 2, 3, 4, 5, 6, 7]
sales = [10, 15, 7, 12, 20, 18, 25]
import matplotlib.pyplot as plt
plt.plot(days, sales)
cursor_line = plt.axvline(x=0, color='gray', linestyle='--')

def on_mouse_move(event):
    if event.xdata is not None:
        cursor_line.set_xdata([event.xdata, event.xdata])
        plt.gcf().canvas.draw_idle()

plt.gcf().canvas.mpl_connect('motion_notify_event', on_mouse_move)
# Define on_click(event) to print clicked coordinates
# Connect on_click to button_press_event
plt.show()

Hint

Use button_press_event to detect clicks and print coordinates.

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